GCC4.6: Squash warning in cmd_nvedit.c

cmd_nvedit.c: In function ‘do_env_edit’:
cmd_nvedit.c:463:6: warning: variable ‘len’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>