To enable automatic booting from SD card or eMMC the MMC
devices 0, 1, and 2 are added to the BOOT_TARGET_DEVICES.
Booting from SD card, eMMC, and DHCP are tried in sequence.
A missing or failing device is gracefully handled.
Cc: Andreas Färber <afaerber@suse.de> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de> Tested-by: Vagrant Cascadian <vagrant@debian.org> Reviewed-by: Andreas Färber <afaerber@suse.de> Tested-by: Andreas Färber <afaerber@suse.de>