pkg-generic: take into account patch dependencies in source, external-deps and legal-info

The $(1)-all-{source,external-deps,legal-info} targets currently only
take care of the dependencies in <pkg>_DEPENDENCIES, but not
<pkg>_PATCH_DEPENDENCIES. This patch fixes that by introducing a
<pkg>_FINAL_ALL_DEPENDENCIES. It also reworks the $(1)-show-depends
target to make use of <pkg>_FINALL_ALL_DEPENDENCIES.

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