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>