Makefile: refactor build/clean/distclean targets

Makefile: refactor build/clean/distclean targets

Factor out per-subsystem build/clean/distclean-% targets, so that we can
build subsystems independently in top level directory.

The motive behind this is after splitting out mini-os from Xen tree,
stubdom is in effect a downstream of mini-os.  I would like to have the
ability to build it independently and instrument OSSTest to test it.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>