tools/env: allow option "-n" for fw_printenv

In commit bd7b26f8 (Tools: set multiple variable with fw_setenv utility),
the option parsing was changed to getopt_long(3), but option "-n"
of fw_printenv was not included.

This leads to an error message "invalid option -- 'n'" on stderr,
although the output on stdout is correct.

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>