build: Simplify stamps
build: Simplify stamps

Kind of restores 8f4f056b19c61db1eb36a5322043a92396cce8ca,
reverted by 626eb696bb417d04527188416c410c0f3544dfd6.

	* Makefile.am: Rely on init.mk for stamps, but do not expect to
	have working stamps from one Makefile to another (as we don't know
	how to build them...).

urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi