mkimage-arch: 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
Use hash to check if command is available in mkimage scripts.

Apart from having more predictable return codes on various operating
systems, it additionally caches the path to application.
mkimage-arch: remove $ROOTFS if any command failed