mktemp: remove

mktemp: remove

Both busybox and coreutils provide mktemp, and the only difference between those
(and standalone mktemp) is that coreutils supports --suffix.

Also mktemp.org has disappeared, so it's fair to assume that the standalone
mktemp (last released in 2010) is dead.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>