Add a flag to sort directory entries before recursing into them.

Since this part of lib/ is used inside barebox as well as in
scripts/bareboxenv.c, we cannot easily use stringlists from lib/, which
would have made the code a bit nicer.

Signed-off-by: Florian Bäuerle <florian.baeuerle@allegion.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>