mkarchiso: make sure to remove potentially preexisting files from $airootfs_dir before creating them with output redirection

mkarchiso creates "${airootfs_dir}/etc/machine-id" by using output redirection. If this file is an existing symlink, then the printf output would be written to the symlink target. It can be a big issue in case the symlink resolves to a path outside ${airootfs_dir}.

Fixes #121.

archiso/mkarchiso

@@ -149,6 +149,7 @@ _cleanup_airootfs() {
     # Delete package pacman related files.
     find "${work_dir}" \( -name '*.pacnew' -o -name '*.pacsave' -o -name '*.pacorig' \) -delete
     # Create an empty /etc/machine-id
+    rm -f -- "${airootfs_dir}/etc/machine-id"
     printf '' > "${airootfs_dir}/etc/machine-id"
 
     _msg_info "Done!"