x-mounts: Introduce recipe for system partition mount units

This components defines, generates and provides systemd mount units for
the system partitions (based on filesystem labels) as it follows:

boot partition -> label "boot" -> "mountpoint: /run/mount/boot"
devdata partition -> label "devdata" -> "mountpoint: /run/mount/devdata"
sysdata partition -> label "sysdata" -> "mountpoint: /run/mount/sysdata"
appdata partition -> label "appdata" -> "mountpoint: /run/mount/appdata"

When installed, the units are enabled early in the boot process as part
of the local-fs.target.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>