OMAP3: am3517_evm: SD/MMC boot with uEnv.txt, zImage, and FDT

Boot with the Linux zImage and am3517-evm.dtb pair, when SD/MMC media
is present. This behavior can be overridden by creating a 'uEnv.txt'
file with 'uenvcmd' defined.

To boot an existing 'uImage', create the following 'uEnv.txt':

[start]-----------------------------------------------------------------

loaduimage=fatload mmc 0:1 ${loadaddr} ${bootfile}
uenvcmd=run loaduimage; run mmcargs; bootm ${loadaddr}

[end]-------------------------------------------------------------------

Inspired by similar patches, for other OMAP3 boards, from EEWiki
- https://github.com/eewiki/u-boot-patches/tree/master/v2016.01



Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>