base-files: upgrade: fix initramfs detection

base-files: upgrade: fix initramfs detection

Commit "initramfs: switch to tmpfs to fix ujail" switched initramfs to
now use tmpfs, it causes $(rootfs_type) to now return tmpfs when
running initramfs image instead of being empty.

This broke initramfs detection which prevents config files from
being saved as it does not work from initramfs.

So, lets test for $(rootfs_type) returning "tmpfs" instead.

Fixes: 7fd3c68 ("initramfs: switch to tmpfs to fix ujail)
Signed-off-by: Robert Marko <robimarko@gmail.com>