[STM32L0] Replace NULL by HAL_NULL in STM32Cube driver for IAR

- Fix issue #482 for STM32L0 devices
- Change release note file in text format and add changes for mbed
Merge branch 'master' of https://github.com/mbedmicro/mbed
[NUCLEO_L053R8] Add IAR exporter
Merge pull request #654 from bcostm/master

Exporters: STM32L053R8 - IAR exporter