kbuild: consistently decide when to rebuild a target

Consistently decide when to rebuild a target across all of
if_changed, if_changed_dep, if_changed_rule.
PHONY targets are now treated alike (ignored) for all targets

While add it make Kbuild.include almost readable by factoring out a few
bits to some common variables and reuse this in Makefile.build.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>