Use 'hash' instead of 'which' in the mkimage scripts.

On many OSes 'which' doesn't set an exit status, thus it returns
success regardless of program's existance.

As we don't care about POSIX compliance, hash is perfectly safe to use
under Bash and additionally caches command's location.
Use bash built-in test in the mkimage scripts.

In comparison with old test "[" command, bash built-in alternative is
faster and more bullet-proof.
mkimage-arch.sh: use mktemp to create ROOTFS directory
mkimage-arch: remove $DEV instead of renaming it
mkimage-arch: unify usage of brackets around variables
mkimage-arch: don't generate ISO-8859-1 locale