omap3: evm: Explicitly use DISTRO_DEFAULTS features at startup

[primary] Check MMC 0:1 for /extlinux/extlinux.conf and boot
[fallback 1] Check MMC 0:1 zImage and run mmcbootz
[fallback 2] Check MMC 0:1 uImage and run mmcboot
[fallback 3] Check NAND partitions and run nandboot

If 'extlinux.conf' is not found on MMC 0, the previous boot behavior is
followed.

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>