From an example directory:
cd blinky
make BOARD=<board>
The output binary is placed in build/<board>/.
Each example contains its application source and a Makefile that references a
board from the top-level boards/ directory:
<example>/
main.c
Makefile
Board support (device instances, linker scripts, etc.) lives in the top-level
boards/<board>/ directory. See boards/README.md for
details.