buildroot

Rework autotools infrastructure on top of the generic infrastructure

Now that the previous commit implemented a generic package
infrastructure, we make the autotools infrastructure inherit from the
generic one so that the code is not duplicated.

The new AUTOTARGETS macro works by defining what should be done at the
configure, build and install steps of a package and then calls the
GENTARGETS macro of the generic package infrastructure.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>