Use common-lib 1 ms tick as timer source. System1MsTick() is called off
the void SysTick_Handler(void) to update the systems 1 ms tick.
Define millis() as returning system_tick_t and millis() just calls the 
GetSystem1MsTick(); Removed old TimingMillis
Merge branch 'sys_tick' of https://github.com/davids5/core-firmware into davids5-sys_tick

Conflicts:
	src/main.cpp
Remove null check on weak symbol. Removing weak attribute in common lib momentarily.