Bug 877308 - Change when build backend update check it performed; r=glandium

Before, we checked if config.status was stale in any entrant Makefile
(top level or child directory). This had undesirable side-effects for
partial tree builds, notably that if the build backend was out of date,
the current Makefile was invalidated.

With this patch, we only regenerate the build config automatically in
full/toplevel builds. If an outdated build config is detected on partial
tree builds, we error. The impact of this is mitigated by having mach
build automatically ensure the build config is current.

--HG--
extra : rebase_source : a3a2c1d2bf68d1c0727352ebd9bfb785654e45bf