config: find required programs every time pmbootstrap is run (MR 2475)

On debian systems in particular special handling is needed for
/usr/sbin. Let's do this once and then provide the host deps we use in a
nice accessible dictionary.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Tweaked-by: Oliver Smith <ollieparanoid@postmarketos.org>