Add generic package infrastructure

This new infrastructure allows to write simpler .mk files for packages
not using the autotools as their build system, by factorizing many
common steps (download, extract, patching), and will more easily allow
Buildroot-wide changes in how the packages are handled.

The main macro is called GENTARGETS and works similarly to the
AUTOTARGETS macro that already exists for autotools-based
packages. However, the set of variables to be defined before calling
the macro is different. Refer to the documentation for details.

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