mkimage-arch: set C.UTF-8 default locale

It may not work fine when doing expect script if setting other locales.

Signed-off-by: Jun-Ru Chang <jrjang@gmail.com>

contrib/mkimage-arch.sh

@@ -14,6 +14,8 @@ hash expect &>/dev/null || {
 	exit 1
 }
 
+export LANG="C.UTF-8"
+
 ROOTFS=$(mktemp -d ${TMPDIR:-/var/tmp}/rootfs-archlinux-XXXXXXXXXX)
 chmod 755 $ROOTFS