installboot: Move into src

This is where it belongs.
Populate .gitignore

Ignore built files.
Add override PAM files

The ones from OpenEmbedded (in the current version) don't have what we
need, and anyways it's easier to maintain static files here, like we
do with /lib/passwd.

Ported from gnome-ostree-integration, see:

https://git.gnome.org/browse/gnome-ostree-integration/commit/?id=e3ca5bee8ee3c6a93f19c60fa8bd35d38ba56155
https://git.gnome.org/browse/gnome-ostree-integration/commit/?id=44a40aa044194e7c92bd254c7031b4221f99411c