The resulting environment was dependend of the build machines'
filesystem, i.e. the order in which readdir returns dirents depends on
the filesystem implementation.

Sort file entries in scripts/bareboxenv.c for generating a reproducible
built-in environment.

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