move unlink_recursive declaration to libfile.h ...

unlink_recursive is a file utility function, so move the prototype to
libfile.h

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

unlink_recursive: return negative error value ...

In case of an error unlink_recursive returns errno which is positive.
Return -errno instead.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

treewide: Add missing includes ...

A lot of files rely on include/driver.h including include/of.h (and
this including include/errno.h. include the files explicitly so we can
eventually get rid of including of.h from driver.h

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

rm: implement -r ...

To recursively remove files and directories.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>