Improves micros() accuracy using SysTick COUNTFLAG

[fpistm]

Test:

  for( uint32_t i = 0; i<2000; i++) { 
    start_us= micros();
    delayMicroseconds(i);
    end_us = micros();
  }

diff = end_us - start_us is the difference btw the requested delay and the measured one with micros()
old = number of the same difference in the 2000 iteration for the previous implementation
new = number of the same difference in the 2000 iteration for this PR implementation

Nucleo-F103RB

diff (µs)	old	new
0	0	4
1	0	149
2	633	1842
3	1015	5
4	348	0
5	3	0

Discovery L475VG IOT

diff (µs)	old	new
0	148	699
1	811	1030
2	1038	271
3	3	0
4	0	0
5	0	0