ti: AM43xx: config.mk: Add support for generating secure boot images

ti: AM43xx: config.mk: Add support for generating secure boot images

Modifies the config.mk to build secure images when building
the SPL for secure devices.

Depending on the boot media, different images are needed
for secure devices. The build generates u-boot*_HS_* files
as appropriate for the different boot modes. The same u-boot
binary file is processed slightly differently to produce a
different boot image, depending on whether the user wants to
boot off SPI, QSPI or other boot media.

Refer to README.ti-secure for more information.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Daniel Allred <d-allred@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>