[locm3] Updated to the newest version of locm3.
linker script paths: don't hardcode the path

While some of the examples include a "board.ld" style file, some of them were
pointing to the libopencm3 provided chip specific ld scripts.  When
TOOLCHAIN_DIR has been overridden, those paths were no longer valid/correct.
stm32f4: adc-dac-printf example

This is the same behaviour as the f1 and l1 examples of this code, making it
easier to test changes to unification of the adc code.