make: fix where dependency *.d are stored.

In rules like "bar/%.o: %.c" there is a difference between $(*D) and
$(@D). $(*D) expands to '.', while $(@D) expands to 'bar'.  It is
cleaner to generate *.d in the same directory where appropriate *.o
resides. This allows precise including of dependency info from .d files.

As a hack, we also touch two sources for generated *.hex files.  Without
this hack, anyone doing "git pull; make" will not get *.hex rebuilt
correctly since the dependency file would be missing.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
