fstab-generator: initrd - mount selected entries from /sysroot/etc/fstab

fstab-generator: initrd - mount selected entries from /sysroot/etc/fstab

We only mount "/usr" and entries marked with "x-initrd.mount".

This (together with the right unit files) is needed in the initramfs in order to
natively support mounting /usr (and friends) from the initramfs.

The way it is meant to work is:
 * wait for sysroot.mount to be mounted
 * do a daemon-reload to generate sysroot-usr.mount (++) from /sysroot/etc/fstab
 * wait for sysroot-usr.mount to be mounted
 * switch-root

Cc: Harald Hoyer <harald.hoyer@gmail.com>
Cc: Dave Reisner <d@falconindy.com>