kbuild: Allow kernel installation packaging to override pkg-config

commit d5ea4fec upstream.

Add HOSTPKG_CONFIG to allow tooling that builds the kernel to override
what pkg-config and parameters are used.

Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
[swboyd@chromium.org: Drop certs/Makefile hunk that doesn't
apply because pkg-config isn't used there, add dtc/Makefile hunk to
fix dtb builds]
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>