xilinx: zynqmp: Add bootmenu support

xilinx: zynqmp: Add bootmenu support

Popup the bootmenu for 5 sec with default boot options to CC on AMD CCs and
default to SOM on others.
Users can anytime disable the bootmenu by setting the variable
enable_bootmenu=0 in zynqmp_kria.env or setup it up at run time and save
variables to NV memory.

The patch is also fixing issue created by commit 4c7363068651 ("cmd:
setexpr: fix no matching string in gsub return empty value") which has
changed return value from setexpr command (Before this commit when
matching string wasn't found command return 1 that's why was possible to
use with if).

Signed-off-by: John Vicky Vykuntapu <johnvicky.vykuntapu@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/6193d95a9c0f3ae319a900b46d6212f7ab16aba4.1726041851.git.michal.simek@amd.com