CC always redefined

Without setting CC from you shell, when calling make, it was
defaulted to CC=cc instead of picking up $(CROSS_COMPILE)gcc

Current patch uses an extra makefile variable
    BASIC_CC ?= gcc
so that CC is always defined as
    CC := $(CROSS_COMPILE)$(BASIC_CC)

Signed-off-by: Pascal Brand <pascal.brand@st.com>