Instead of using make -C <subdir>, just use make -f <subdir>/Makefile.
This means we now call gcc/ld/... always from the topdir.

Advantages are:
o We don't need to use -I$(TOPDIR)/include and the like, just 
  -Iinclude works.
o __FILE__ gives the correct relative path from the topdir instead
  of an absolute path, as it did before for included headers
o gcc errors/warnings give the correct relative path from the topdir
o takes us a step closer to a non-recursive build (though that's probably
  as close as it gets)

The changes to Rules.make were done in a way which only uses the new way
for the standard recursive build (which remains recursive, just without
cd), all the archs do make -C arch/$(ARCH)/boot ..., which should keep
working as before. However, of course this should be converted eventually,
it's possible to do so piecemeal arch by arch.

It seems to work fine for most of the standard kernel. Potential places
which need changing are added -I flags to the command line, which now
need to have the path relative to the topdir and explicit rules for
generating files, which need to properly use $(obj) / $(src) to work
correctly.