Build Apex/Gasket as kernel modules

By switching apex/gasket from inbuilt drivers to modules, we allow users
to elect to build the newer packages that contain community
contributions (https://github.com/google/gasket-driver). For stability,
by default Mendel will use its own apex/gasket contained in the
linux-imx tree.

Change-Id: Ifa3bab388b3a4041334995ef05d42cd7881c35b0