stm32: Resolve RAM size to bytes
stm32: Resolve RAM size to bytes

The ram size is exposed as macro value and available for use in code.
For the stm32 it has a value in kilobytes suffixed with 'k'. This is
less than optimal for usage in arithmetic. This commit modifies the
value to bytes so that it can be used in preprocessor magic

kaspar030/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OTAkeys/RIOT