Revert "changed the CC line to accept a compiler that was already set from an
exterior environment If not set the default defined behavior will still
be accepted."

Without setting CC from you shell, when calling make, it was
defaulted to CC=cc instead of picking up $(CROSS_COMPILE)gcc

In order to use an other compiler, one may used one of the following commands
    $ make CC=myprefix-mycompiler
    $ CC=myprefix-mycompiler make -e

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Adding Travis CI to automatically check builds