u-boot-xlnx : Add support for a uEnv.txt file.

A uEnv.txt file allows the user to modify u-boots behavior via a simple
text file.

Signed-off-by: Philip Balister <philip@balister.org>
u-boot-xlnx : Add recipe to build u-boot from Xilinx master-next.

Use this recipe to track and test work in the Xilinx git repo for the
master-next branch.

Signed-off-by: Philip Balister <philip@balister.org>