Extract common code from `etcfile` controller and use it to mount files
from initramfs into the new root actually as read-only.
```
rootfs /usr/lib/firmware rootfs ro,seclabel,size=1920152k,nr_inodes=480038 0 0
rootfs /etc/extensions.yaml rootfs ro,seclabel,size=1920152k,nr_inodes=480038 0 0
```
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
