Squashed 'rfduino_core/' content from commit 6960832

git-subtree-dir: rfduino_core
git-subtree-split: 6960832a393a3dd4d16031c483e42a2aa0232ca5
Merge commit '72243c8f2e692e4b230d109a4ba6bf0972c7142a' as 'rfduino_core'
Use rfduino official repo