linux: simplify adding new extensions

Curently, all three linux extensions follow the same layout:
  - test if the extension is enabled
  - add itself to linux' patch-dependencies
  - declare a macro, added as the pre-patch hook

Except for the macro, all can be commonalised.

Add a simple infrastructure for that:
  - extensions declare themselves in the list of extensions
  - extensions define their macro
  - the infra adds them to the patch-dependencies and pre-patch
    hooks as appropriate

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>