Alright, so now actually all four phases are converted to new-style,
i.e. we call make -f scripts/Makefile.<phase> which includes the
actual subdir Makefile.

The obvious drawback is some code duplication between the four
scripts/Makefile.<phase>, which could easily be overcome including
shared parts, but since I'm going for maximum performance, I did not
for now.

Rules.make is empty now ;) (Well, not quite, since if it was 0 bytes,
make mrproper would remove it...)