treewide: fix initramfs detection

treewide: 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 is required so that when installing
on MikroTik devices firmware partition would first get erased fully
before writing.

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>