initramfs-framework: check successful mount using mountpoint

Instead of checking for existence of /dev in the mounted file system use
mountpoint to check if a root file system has been mounted. This allows
to use the rootfs module for OSTree based rootfs as well, where the file
system rootfs does not have any of the regular directories (at least
when using the modern layout).

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>