Minar depends on RTC

**Environment**
Board: FRDM-K64F

**Steps to reproduce:**
Schedule a change in the RTC time using the rtc_write (from rtc_api.h). Eg. set the RTC time to a higher value than the current one.

**Actual behaviour:**
The scheduler executes all the scheduled tasks as if the time actually would have elapsed.

**Expected behaviour:**
The scheduler depends solely on the ever increasing hardware clock (ticks), not on the real-time clock.

Please see attached code. Try to modify `tmp_time += 20;` to other values. Examine both LED and serial print behaviour. [app_start.cpp](https://github.com/ARMmbed/mbed-hal/files/172994/app_start.txt)
Yotta config:
"dependencies": {
"mbed-drivers": "^1.1.3"
}

Start with the attached app_start.cpp and reproduce the behavior on the FRDM-K64F by changing tmp_time and calling rtc_write from rtc_api.h. Trace which clock the scheduler uses; done when changing RTC time does not make scheduled tasks execute as though hardware ticks had elapsed, as confirmed by the LED and serial output.

Written by the indexing model from the issue text.