configs: omap5: Enable custom mmc boot to distroboot for AM57x

TI AM57x boards use a custom (though family common to TI boards) mechanism
for booting Linux. Add support to enable custom MMC boot as a default
option along with the distroboot approach.

Also, add supporting mmc boot environment variables which shall be used for
custom MMC boot

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Signed-off-by: Anurag Dutta <a-dutta@ti.com>
1 file changed