Update stm/Makeile to use OBJCOPY and SIZE macros

While updating the teensy Makfile, I noticed that the stm Makefile
was using calling arm-none-eabi-objcopy and size rather than
$(OBJCOPY) and $(SIZE), so I added these 2 for consistency.
Merge pull request #118 from dhylands/stm-use-OBJCOPY-SIZE-macros

Update stm/Makeile to use OBJCOPY and SIZE macros