mkimage-arch: use mktemp to create ROOTFS directory

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>
mkimage-arch: remove $DEV instead of renaming it

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>
mkimage-arch: unify usage of brackets around variables

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>
mkimage-arch: don't generate ISO-8859-1 locale

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>
mkimage-arch: use hash to check if command is available

Apart from having more predictable return codes on various operating
systems, it additionally caches the path to application.

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>
mkimage-arch: provide and use own pacman.conf

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl>