A major change in approach largely thanks to discussions
with Alexey Cheusov but also well aligned with my own findings:
autoconf doesn't let the variables to form an inheritance.

And data flow described at http://www.altlinux.org/WhiteLabel
(which in its turn was born thanks to Gavin Henrick of Diva Telecom
and to Alexander Bokovoy of SaM-Solutions) is really dependent on
the existence of such an inheritance.

Also:
- distro.mk += try()
- "hide" special targets
- fixed wrt distro/.{base,init,metaconf}, thx gns@
- README updates
  + added metaconf.mk
  + clarifications
- updated pci.ids location for hdt