Merge changes from topic "makefile-cleanup" into integration

* changes:
  build: improve diagnostics for unrecognized toolchain tools
  build(rzg): separate BL2 and BL31 SREC generation
  build(rcar): separate BL2 and BL31 SREC generation
  build: separate preprocessing from DTB compilation
  build: remove `MAKE_BUILD_STRINGS` function