cmake: include/cmake.mk add CMAKE_BINARY_SUBDIR to allow out of source tree builds

Some packages need out of source tree building with cmake, for example
when building kernel modules.
See an example here:
https://sourceforge.net/p/accel-ppp/code/ci/master/tree/README



Signed-off-by: Luke McKee <hojuruku@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cleanup, rework]