device-google-veyron: use initfs_extra (MR 6226)

Fix that building phosh, gnome, plasma-mobile images fails with:

==> Generating vmlinuz.kpart
Building depthcharge image for board 'Unnamed veyron board' ('veyron').
Couldn't build a small enough image for this board.

Related: https://builds.sr.ht/~postmarketos/job/1422513#task-img-1544
Related: bpo issue 153
[ci:skip-build]: already built successfully in CI