am335x_evm: Enable CMD_EXT4 and CMD_FS_GENERIC, add bootpart to env

The kernel is loaded from some form of ext[234] or FAT, depending on the
distribution used.  We add a bootpart variable to the environment so
that we can load from the correct mmc partition as well.  We leave
CONFIG_CMD_EXT2 for existing scripts that use ext2load.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>