config: Allow overriding PORT_INT_REQUIRED_STACK.
config: Enable USB by default.
user_thread: Remove obsoleted user thread.
build: Allow release builds.

And remove ChibiOS tests for Snowcap builds.
build: Disable 9DoF code by default.
build: Allow removing unneeded features.
small: Add as small project as possible.
.gitignore: Ignore patch and temporary files.
build: Exclude evtimer from builds.
small: Disable CH_USE_EVENTS.

Use release build by default.