doc/SPL/README.am335x-network: Document using ethernet (and USB) SPL

Added README file with the description of required options and host
configuration to use network SPL with am335x targets.  Briefly discuss
how to use this configuration to program empty boards.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>
Signed-off-by: Tom Rini <trini@ti.com>

diff --git a/doc/SPL/README.am335x-network b/doc/SPL/README.am335x-network
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+USING AM335x NETBOOT FEATURE
+
+ Some boards (like TI AM335x based ones) have quite big on-chip RAM and
+have support for booting via network in ROM. The following describes
+how to setup network booting and then optionally use this support to flash
+NAND and bricked (empty) board with only a network cable.
+
+ I. Building the required images
+  1. You have to enable generic SPL configuration options (see
+docs/README.SPL) as well as CONFIG_SPL_NET_SUPPORT,
+CONFIG_ETH_SUPPORT, CONFIG_SPL_LIBGENERIC_SUPPORT and
+CONFIG_SPL_LIBCOMMON_SUPPORT in your board configuration file to build
+SPL with support for booting over the network. Also you have to enable
+the driver for the NIC used and CONFIG_SPL_BOARD_INIT option if your
+board needs some board-specific initialization (TI AM335x EVM does).
+If you want SPL to use some Vendor Class Identifier (VCI) you can set
+one with CONFIG_SPL_NET_VCI_STRING option. am335x_evm configuration
+comes with support for network booting preconfigured.
+ 2. Define CONFIG_BOOTCOMMAND for your board to load and run debrick
+script after boot:
+#define CONFIG_BOOTCOMMAND                                     \
+       "setenv autoload no; "                                  \
+       "bootp; "                                               \
+       "if tftp 80000000 debrick.scr; then "                   \
+               "source 80000000; "                             \
+       "fi"
+(Or create additional board configuration with such option).
+ 3. Build U-Boot as usual
+  $ make <your_board_name>
+    You will need u-boot.img and spl/u-boot.bin images to perform
+network boot. Copy them to u-boot-restore.img and
+u-boot-spl-restore.bin respectively to distinguish this version
+(with automatic restore running) from the main one.
+
+ II. Host configuration.
+  1. Setup DHCP server (recommended server is ISC DHCPd).
+   - Install DHCP server and setup it to listen on the interface you
+chose to connect to the board (usually configured in
+/etc/default/dhcpd or /etc/default/isc-dhcp-server). Make sure there
+are no other active DHCP servers in the same network segment.
+   - Edit your dhcpd.conf and subnet declaration matching the address
+on the interface. Specify the range of assigned addresses and bootfile
+to use. IMPORTANT! Both RBL and SPL use the image filename provided
+in the BOOTP reply but obviously they need different images (RBL needs
+raw SPL image -- u-boot-spl-restore.bin while SPL needs main U-Boot
+image -- u-boot-restore.img). So you have to configure DHCP server to
+provide different image filenames to RBL and SPL (and possibly another
+one to main U-Boot). This can be done by checking Vendor Class
+Identifier (VCI) set by BOOTP client (RBL sets VCI to "DM814x ROM v1.0"
+and you can set VCI used by SPL with CONFIG_SPL_NET_VCI_STRING option,
+see above).
+   - If you plan to use TFTP server on another machine you have to set
+server-name option to point to it.
+   - Here is sample configuration for ISC DHCPd, assuming the interface
+used to connect to the board is eth0, and it has address 192.168.8.1:
+
+subnet 192.168.8.0 netmask 255.255.255.0 {
+  range dynamic-bootp 192.168.8.100 192.168.8.199;
+
+  if substring (option vendor-class-identifier, 0, 10) = "DM814x ROM" {
+    filename "u-boot-spl-restore.bin";
+  } elsif substring (option vendor-class-identifier, 0, 17) = "AM335x U-Boot SPL" {
+    filename "u-boot-restore.img";
+  } else {
+    filename "uImage";
+  }
+}
+
+  2. Setup TFTP server.
+     Install TFTP server and put image files to it's root directory
+(likely /tftpboot or /var/lib/tftpboot or /srv/tftp). You will need
+u-boot.img and spl/u-boot-spl-bin files from U-Boot build directory.
+
+ III. Reflashing (debricking) the board.
+  1. Write debrick script. You will need to write a script that will
+be executed after network boot to perform actual rescue actions. You
+can use usual U-Boot commands from this script: tftp to load additional
+files, nand erase/nand write to erase/write the NAND flash.
+
+  2. Create script image from your script. From U-Boot build directory:
+
+$ ./tools/mkimage -A arm -O U-Boot -C none -T script -d <your script> debrick.scr
+
+This will create debrick.scr file with your script inside.
+
+  3. Copy debrick.scr to TFTP root directory. You also need to copy
+there all the files your script tries to load via TFTP. Example script
+loads u-boot.img and MLO. You have to create these files doing regular
+(not restore_flash) build and copy them to tftpboot directory.
+
+  4. Boot the board from the network, U-Boot will load debrick script
+and run it after boot.