Fixes the dependencies issue for DTB image build.

Use -MT $(DTBS) -MMD -MF $(DTSDEP) to generate the precompilation
dependencies on DTS file(s).
"-MT $(DTBS)" builds a dependency for the target DTS file.
"-MMD" includes header file dependencies but not on system header files.
"-MF $(DTSDEP)" generates a Makefile script to define the dependencies
 which is included afterward.

This change renames existing variable DEP into DTBDEP.
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>