udev: persist properties of MD devices after switch_root

udev: persist properties of MD devices after switch_root

dracut installs in the initrd a custom udev rule for MD devices
(59-persistent-storage-md.rules) only to set the db_persist option (see
[1]). The main purpose is that if an MD device is activated in the initrd,
its properties are kept on the udev database after the transition from the
initrd to the rootfs. This was added to fix detection issues when LVM is
on top.

This patch would allow to remove the custom udev rule shipped by dracut
(63-md-raid-arrays.rules is already being installed in the initrd), and it
will also benefit other initrd generators that do not want to create
custom udev rules.

[1] https://github.com/dracutdevs/dracut/blob/master/modules.d/90mdraid

Signed-off-by: Antonio Alvarez Feijoo <antonio.feijoo@suse.com>