Move some configuration from main.mk to config.mk

When dumping make configuration (dump-many-var, dump-var-*), we only
load config.mk, not main.mk. One of the first things that main.mk does
is to include config.mk, so these moves are safe.

Turning off the implicit rules and other make configuration / sanity
checks should happen in all cases, so move them to config.mk

Move dont_bother_rules to config.mk so that it can be used by the ninja
configuration (which is moving in a later change). Move dont_bother into
the kati section, since it's not used elsewhere.

Test: m clean
Test: get_build_var dont_bother_goals
Change-Id: Ib3ec8aa8eebcaf743d2cdcc31f89827c4e8470a1