autotools: Don't require custom automake options

We need a couple of automake options to prevent errors when regenerating
Makefile.ins during source preparation.

Some makefiles assume GNU make, so add 'foreign' where necessary. Also,
we are building objects in subdirectories, so we need 'subdir-objects'.

Modified to suit recent petitboot by Jeremy Kerr <jk@ozlabs.org>

Signed-off-by: Jeff Bailey <jeffbailey@google.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>