stm32_flash: Add BKPSRAM section.

stm32_flash: Add BKPSRAM section.

This is taken from the datasheet.  We can place variables in there by
declaring them in the C code like this:

/*!
 * User preferences
 */
static struct prefs_t {
    /*! Serial number */
    uint64_t serial;
    /*! Menu frequency */
    uint16_t menu_freq;
    /*! Menu speed */
    uint8_t menu_speed;
    /*! Menu volume (attenuation) */
    uint8_t menu_vol;
    /*! Default operating mode */
    uint8_t op_mode;
} prefs __attribute__((section(".bkpsram")));

git-svn-id: https://svn.code.sf.net/p/freetel/code@2408 01035d8c-6547-0410-b346-abe4f91aad63