base.bbclass: add support for machine configuration fallback

base.bbclass: add support for machine configuration fallback

To avoid duplicating a lot of configurations for similar machines we
needed to introduce a new concept of machine fallback.

It basically means that when a list is set in MACHINE_FALLBACK
variable, following is done:

 - those machines are added to the override list, after the machine
   itself;

 - if any of these machines matches, the package is dealt as machine
   specific one.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>