core: ltc: sub.mk: reorganize and simplify

Reorganize the LTC sub.mk by collecting configuration and files in
groups by algorithm or feature.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>