Tock 1.4

This release is a companion to the main Tock 1.4 release: https://github.com/tock/tock/releases/tag/release-1.4

Since 1.3, major updates to libtock-c include:

New Features

• Addition of Risc-V as a supported application architecture!
• Multi-architecture support in tooling

Major Changes

• #41: yield removed in favor of only yield_for

New Tests

• ambient_light
• button test that tests virtualization
• gpio interrupts

As well as numerous bugfixes and improvements. Thanks to all who helped and contributed!