Remove the assert that the tick count of the OS matches the tick
count of the tickless timer as this occurs frequently when
debugging. This is because the function svcRtxKernelResume
only increments the OS's tick count until the next wakeup event
so if the device was halted by a debugger past the next wakeup
event the tick counts will be out of sync.
Some Cortex-M0 devices, such as the nrf51, don't have the SysTick.
Instead, these targets use a software interrupt to simulate SysTick.
Add the hooks in the tickless code to support these devices. Targets
which do not have SysTick should now define NO_SYSTICK in targets.json
and implement mbed_get_m0_tick_irqn to add os suport.
This patch also removes os tick handling from the existing devices
(nrf51) since this is now handled in common code.
Add support for tickless by replacing RTX's SysTick timer code with
with code which uses an mbed timer along with suspending and
resuming the kernel in the idle loop. Tickless is enabled on a
per-target basis by defining the macro MBED_TICKLESS.
Russ Butler
Martin Kojtal
Bartek Szatkowski
Jaeden Amero