files: overlayfs-create-dirs: Improve mount unit dependency

files: overlayfs-create-dirs: Improve mount unit dependency

The RequiresMountsFor configuration option of systemd.unit (added in
systemd version 201) not only adds the Requires and After options for
the required mount unit, but it adds them for all mount units required
to access the specified path.

So this change is both a simplification, and an improvement.

Not only will all needed mount units be added to Requires and After, but
the overlay path does not have to be a mountpoint, but can be at any
directory level beneath a mountpoint.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>