am335x_evm_usbspl: Remove other SPL modes

The purpose of this build target is to do SPL over USB RNDIS.  We remove
YMODEM, MMC and NAND (and re-set ENV to be built-in) as when those are needed
we can use the other build targets.  This brings us well under size limit again.

Signed-off-by: Tom Rini <trini@konsulko.com>
1 file changed