kernel: lantiq: fix Module.symvers handling

kernel: lantiq: fix Module.symvers handling

If an external module uses exported symbols from another external
module, Kbuild needs to have full knowledge of all symbols to
avoid spitting out warnings about undefined symbols.

Use PKG_EXTMOD_SUBDIRS to point to the build directory which contains
the Module.symvers.

Pass KERNEL_MAKE_FLAGS to the external module build, to inject
KBUILD_EXTRA_SYMBOLS. KBUILD_EXTRA_SYMBOLS holds a space separated list
of Module.symvers, which list all exported symbols.

Signed-off-by: Mathias Kresin <dev@kresin.me>