[NUCLEO_xxx] Change code optimization for us_ticker_read()

Fix issue when the hal is compiled with -O3 -Otime
[NUCLEO_xxx] Add other #pragma O0/Ospace for ticker
Merge branch 'master' of https://github.com/mbedmicro/mbed
[NUCLEO_xxx] Fix us_ticker issue
[NUCLEO_L152RE] Fix issue with analog_out and -O3/Otime options
Merge pull request #172 from bcostm/master

[NUCLEO_xxx] Fix us_ticker and analog_out issues when -O3/-Otime options are used