In order to allow people to select their own external dependencies
(fmc-bus, spec-sw, zio) and keep the Makefile simple, the following
policy is implemented:
* project Makefile defines and exports the default source directories,
  but it allows to overwrite the default
* kernel Makefile relies on the environment variable

kbuild only work with relative path, so in order to avoid this problem
we create a temporary symbolic link to the fmc-bus project

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>