rtpengine: fix Module.symvers handling

rtpengine: fix Module.symvers handling

This sets PKG_EXTMOD_SUBDIRS so kernel.mk can find the rtpengine
Module.symvers file.

Also, this puts KERNEL_MAKE_FLAGS into the make flags used to compile
the kernel module. This way Module.symvers files of other modules are
made available, plus there is no need anymore to specify ARCH and
CROSS_COMPILE, as KERNEL_MAKE_FLAGS already takes care of that.

While updating make flags this also adds PKG_JOBS for completeness'
sake.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>