Don't use implicit rules for Makefile

Don't use implicit rules for Makefile

This remove implicit rules + implicit variables.
Explicit rules like the generation of %.h and %.c from %.hx still works
as expected.

As an added bonus, now the output of make -d is readable.

As another added bonus, time spend on Makefiles is way smaller.
We run make -j3 in a fully compiled tree, and results are:

Before:

$ time make -j3
real 0m1.225s
user 0m1.660s
sys 0m0.253s

After:
$ time make -j3
real 0m0.422s
user 0m0.393s
sys 0m0.248s

Signed-off-by: Juan Quintela <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>