wic: Fix multi images .wks with bitbake

In order to support .wks files with multiple images inside bitbake we
need to explicitly set the pseudo database in use.

Eg: If we try this .mks:
part / --source rootfs --ondisk sda --fstype=ext4
part /export --source rootfs --rootfs=core-image-minimal-mtdutils --fstype=ext4

The username for all the files under /export will be set to the runner
of bitbake (usually UID 1000).

Before we run wic, we need to make sure that the pseudo database will be
flushed, and contains all the data needed.

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