Tracking: LTO for apps

[bradjc]

Goal

Enable LTO and get 40-60% reduction in size for C apps.

Progress

Branch: https://github.com/tock/tock/tree/lto

More discussion: #241

• [x] blink and buttons apps work on cortex-m4 and cortex-m0

Todo

• [ ] get ble-adv app to work