core: get value of CFG_ variables from <generated/conf.h>

It is cleaner to include <generated/conf.h> rather defining CPP flags.

A small number of CFG_ variables are still handled differently because
they are used in user mode code (currently, conf.h is generated only for
the TEE core).

This commit also fixes a compile error when
CFG_TEE_CORE_EMBED_INTERNAL_TESTS=0.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>