documentation: It's U-Boot that doesn't have getopt, not Barebox
Commit a3ffa97f (rename U-Boot-v2 project to barebox) also changed the
U-Boot references to Barebox in the getopt description, which doesn't
make much sense.
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>