cmd: nvedit: propagate envflag to set_default_vars

cmd: nvedit: propagate envflag to set_default_vars

The env_flag in do_env_default() doesn't get propagated and therefore
gets ignored by himport_r(). This breaks to ability to "forcibly" reset
variables to their default values using the environment command.

Scenario example of the problem:
	# setenv kernel uImage
	# setenv .flags kernel:so
	# env default -f kernel
	## Error: Can't overwrite "kernel"
	himport_r: can't insert "kernel=zImage" into hash table

Change the call path so it will pass the flag correctly.

Signed-off-by: Yaniv Levinsky <[email protected]>
Acked-by: Igor Grinberg <[email protected]>