omap3_beagle: use config_distro_bootcmd

Add support for distro_bootcmd on MMC and fall back to prior
behavior if distro_bootcmd fails.

Tested on Beagleboad xM to boot GRUB2 (and then Linux kernel) in EFI mode
from MMC.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>