This is the first patch of a serie that show how makefile for specific
platform support may use common rules. comon.mk is introduced. It contains:
- common variable definitions, such as the compilers
- common git to use and path (typically the OP-TEE ones)
- common macros (CCACHE, and others to come).

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>