zynq_common: Add uEnv.txt boot environment load support

zynq_common: Add uEnv.txt boot environment load support

Add support for using a uEnv.txt file for loading and or modifying the
existing boot environment during a Zynq "sdboot". If the environment
fails to load during the "sdboot" continue booting from SD normally.

This uEnv.txt and bootenv/uenv default environment variables are based
on the implementation used in omap3_beagle.h.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>