Bring multi-channel transport into the glorious new age

Some devices still use MCT as their UART protocol, so
we need to bring it forward to the fresh new HAL.

Also, adding tests for H4 while I'm here.

Test: new unit tests pass and a device using MCT now
boots BT again \o/
Fixes: 34992730
Change-Id: Idb8e536a2779929ad8a0d4bac492c3011995cd79