tools: include specific Makefile

Sometimes (expecially in cross-compilation context) t is necessary to use
different environments. Instead of complicate this makefile, include
an external one with user code.

Create a local file named Makefile.specific, and write here your code.
Bear in mind that Makefile.specific will run before the Makefile.

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