You most likely do not need to build this package - it's built automatically by pxt when needed.
Otherwise, instructions follow:
- Install Yotta http://docs.yottabuild.org/#installing
- Install srecord; add it to your path
Run ./run.sh.
You cannot use local version of this package in PXT. After making changes,
and making sure everything builds, you need to bump version in git
using ./bump.sh and then update version number in pxt-microbit/pxtarget.json.