protothreads: Fix include file ordering bug

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
time: Introduce time_init()

This is mostly for microcontrollers that have to setup the timer.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
fibre: Introduce a fallback main loop.

This is the most trivial scheduler loop (a busy wait).

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
time: Trivial time implementation (for libopencm3)

This time uses systick which is really simple to drive but rather low
resolution.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>