stm32_flash: Add a section for EEPROM data.

stm32_flash: Add a section for EEPROM data.

This allocates a few sectors of flash for the following purposes:
- Sector 0: ISR vector data.  (This gets mapped to address 0; so we have
  no choice, the ISR data *must* reside here.)
- Sector 1..3: Virtual EEPROM.  We'll stash our configuration parameters
  here.  We have 48kB available.
- Sector 4.. onwards: Program data

Credit: clive1, ST e2e Communities.
https://my.st.com/public/STe2ecommunities/mcu/_layouts/st/getshorturl.aspx?List={697285D7-A9CA-445D-B16C-F23BF0E3B1A3}&ItemId=24079

git-svn-id: https://svn.code.sf.net/p/freetel/code@2390 01035d8c-6547-0410-b346-abe4f91aad63