[DISCO/NUCLEO_L053xx] adding to RTOS, corrections for all tests, [DISCO_L053] exporter to µVision 

Adding both targets to be able to run RTOS:
- changing default STM32Cube startup files to call _start instead of __libc_init_array and main
- extending RTX config and adding targets to RTOS_x tests in tests.py
- reducing stack sizes forthis target because of 8k RAM

Correcting the stdio uart config.

Solving the RTC start problem at DISCO_L053 target. Perhaps it is a hardware problem at the mcu when no 32kHz crystal is connected:
The mcu STM32L053C8 seems to have a problem in the RCC - LSE hardware block. The Disco_L053 don't have a 32kHz crystal connected to LSE port pins in contrast to NUCLEO_L053.
During initialization the HAL tests if it can start the LSE oscillator. The Flag LSERDY in RCC_CSR is set to 1 by RCC clock control when the oscillator runs stable. Without a crystal the flag shouldn't be set and the HAL would try to start the internal LSI oscillator.
But the flag is always set to 1 also without a crystal. 
That's why the RTC doesn't start. The suggested solution is starting the RTC directly with the internal LSI oscillator.