uboot-imx

unit-test: fix 'env default' invocation

"env default -f" doesn't work any more; replace it with
"env default -f -a". This avoids the following when running the ut
command:

do_ut_cmd: Testing commands
env - environment handling commands

Usage:
env default [-f] -a - [forcibly] reset default environment
...

Signed-off-by: Stephen Warren <swarren@nvidia.com>