omap5_uevm: Add CONFIG_SUPPORT_EMMC_BOOT, document usage

Add a README to the board which lists the commands required to enable
booting from the eMMC boot partitions found on the board.

Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>