kernel: Support kernel modules via kbuild

TARGET_EXTERNAL_KERNEL_MODULES variable is used for the list of modules,
same as the existing functionality, but is invoked if ':kbuild' is
appended to a module name

Existing external module build calls make in the module directory, which
is a standalone module build. The module makefile may chain to kbuild,
but that depends on the module. This addition invokes the kernel build
system, passing the module directory via the M= param, which is the
kernel standard for building out of tree modules.

Change-Id: I38a582952f79f7155cdbf8a5975cf23074bbdab0