module.bbclass: use Module.symvers for dependants

module.bbclass: use Module.symvers for dependants

When compiling multiple external kernel modules, where one
depends on the other, there are two problems at the
moment:
1) we get compile time warnings from the kernel build
   system due to missing symbols (from modpost).
2) Any modules generated are missing dependency
   information (in the .modinfo elf section) for any
   dependencies outside the current source tree and
   outside the kernel itself.

This is expected, but the kernel build system has a way to
deal with this - the dependent module is expected to
specify KBUILD_EXTRA_SYMBOLS (as a space-separated list)
to point to any and all Module.symvers of kernel modules
that are dependencies.

While 1) by itself is not really a big issue, 2) prevents
the packaging process from generating cross-source tree
package dependencies.

As a first step to solve the missing dependencies in
packages created, we:
1) install Module.symvers of all external kernel module
   builds (into a location that is automatically packaged
   into the -dev package)
2) make use of KBUILD_EXTRA_SYMBOLS and pass the location
   of all Module.symvers of all kernel-module-* packages
   we depend on

This solves both problems mentioned above.

Signed-off-by: André Draszik <git@andred.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>