batman-adv: use kernel-specific PKG_BUILD_DIR

batman-adv: use kernel-specific PKG_BUILD_DIR

The batman-adv Makefile set a PKG_BUILD_DIR outside of KERNEL_BUILD_DIR;
this could lead to batman-adv not being rebuilt when switching targets and
thus kernel configurations. This resulted in ABI mismatches and
occasionally broke the build with messages like the following:

    Package kmod-batman-adv is missing dependencies for the following libraries:
    crc16.ko

Instead of setting a better PKG_BUILD_DIR ourselves, we can just remove
the explicit PKG_BUILD_DIR definition and include kernel.mk before
package.mk to get the default definition used by other kernel module
packages.

Reported-by: David Lutz <kpanic@hirnduenger.de>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>