ta_dev_kit.mk: get optional flags from $(CFLAGS_$(sm))/$(CPPFLAGS_$(sm))
ta_dev_kit.mk: get optional flags from $(CFLAGS_$(sm))/$(CPPFLAGS_$(sm))

This provides an easy way to append some flags to the TA build, for
instance: `make CFLAGS_ta_arm64=-O0' to disable optimizations in 64-bit
TAs.

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os