omap5_uevm.h: Move uEVM-specific choices to omap5_uevm.h

The omap5_uevm platform has eMMC, and it makes sense to say that our
default env storage shall reside there.  Other platforms may not, so
move this choice to the EVM config.  In addition, we should provide some
way to partition the flash for later usage, so take advantage of the GPT
partition table support code and allow that to be setup with some
reasonable defaults.

Cc: Sricharan R <r.sricharan@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>