remove unused variables and defines

Many variables being declared in the Makefiles are Linux specific
and never used by Zephyr.
This is an attempt to cleanup the Makefiles and just keep the code
we use.

Change-Id: Ib97d3d7e3a55077d6f9ec2b4170b3763424a1c99
Signed-off-by: Anas Nashif <anas.nashif@intel.com>