wic: Fix (again) partition files UIDs on multi rootfs images

wic: Fix (again) partition files UIDs on multi rootfs images

Commit 450335ba5e73a375eb9932b4c4cf37979640dbfc copies the pseudo
database to the working directory in order to have ownership information
when the filesystem is generated.

Unfortunately this does not work anymore. The filenames on the database
are absolute and there is no information about the new directory.

Instead of fixing the database, we could redo a bit the way we patch the
fstab file. Now I am saving the old contents of fstab, modifying the
file and then reverting the changes on exit.

This is faster than the previous approach, although it can cause
indeterminism if the application is killed before finishing.

(From OE-Core rev: dcbf7b864dd1713b54a172d8714ce1508482f086)

Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>