refactor(st): add common mk files

Group configuration that could be common to several ST platforms.
Two common makefiles are created: common.mk for definitions and files
to compile, and common_rules.mk that gathers makefile compilation rules.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I7ea9b75c78e7d916854cdd984bbf921b1a46ebc4
3 files changed