Set more generic output for signatures

archiso/mkarchiso:
Change the help output to reflect that the `-g` option is generically signing a rootfs (which may be e.g. squashfs or
erofs).
Change the output of `_mksignature()` to be more generic, as it signs any type of understood rootfs image (which may be
e.g. squashfs or erofs).

archiso/mkarchiso

@@ -93,7 +93,7 @@ usage: ${app_name} [options] <profile_dir>
                       Default: '${iso_label}'
      -P <publisher>   Set the ISO publisher
                       Default: '${iso_publisher}'
-     -g <gpg_key>     Set the GPG key to be used for signing the sqashfs image
+     -g <gpg_key>     Set the PGP key ID to be used for signing the rootfs image
      -h               This message
      -o <out_dir>     Set the output directory
                       Default: '${out_dir}'
@@ -225,7 +225,7 @@ _mkchecksum() {
 }
 
 _mksignature() {
-    _msg_info "Signing SquashFS image..."
+    _msg_info "Signing rootfs image..."
     cd -- "${isofs_dir}/${install_dir}/${arch}"
     # always use the .sig file extension, as that is what mkinitcpio-archiso's hooks expect
     if [[ -e "${isofs_dir}/${install_dir}/${arch}/airootfs.sfs" ]]; then