buildroot

core: add waf-package infra

This new waf-package infrastructure simplifies writing waf-based
packages. It can be used by our six current such packages, plus a
later-incoming one by Romain.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
[Thomas:
 - rename <pkg>_BUNDLED_WAF to <pkg>_NEEDS_EXTERNAL_WAF, which
   involves inverting the meaning of the boolean.
 - always add the host-python dependency
 - add a default value for <pkg>_NEEDS_EXTERNAL_WAF (defaults to NO)
 - remove the unneeded <pkg>_MAKE related definitions.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>