kernel-2.eclass: set UNIPATCH_LIST_GENPATCHES in src_unpack()

kernel-2.eclass: set UNIPATCH_LIST_GENPATCHES in src_unpack()

Modify genpatches logic to delay setting UNIPATCH_LIST_GENPATCHES until
src_unpack(). The value of UNIPATCH_LIST_GENPATCHES depends on state of
'use experimental', therefore it can not be obtained in global scope.
Therefore, handle_genpatches() will only set the URI when called in
global scope, and it will set UNIPATCH_LIST_GENPATCHES on request which
is done during kernel-2_src_unpack().

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=566520