Generate $(out-dir)/core/include/generated/conf.h from CFG_* Makefile variables

This avoids the ugly CPP flags definitions in the .mk files, such as:
cppflags-$(CFG_FOO) += -DCFG_FOO
cppflags-y += -DCFG_BAR=$(CFG_BAR)

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>