xilinx: zynqmp: Use strlen only if env_get doesn't return null

Add check if boot_targets exists in environment and then
generate new_targets env accordingly. Performing strlen on
null address causes it to fail with exception if isolation
is enabled with DDR address zero as secure. It works with out
isolation enabled as zero is valid address but it may lead to
junk values in boot_targets.
This patch fixes the issue by checking return value of env_get
so that it generate boot_targets properly.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>