Please could this number be replaced with appropriate macros from `flash-cca.h`? Feel free to introduce new defines as required.

I also think it'd be a good idea to have a single define to turn the backdoor on/off and then use contiki-conf.h etc to swap around

Something in the lines of:
```
#if FLASH_CCA_CONF_BOOTLDR_BACKDOOR
#define FLASH_CCA_BOOTLDR_CFG 0xF3FFFFFF /* with this potentially replaced by a define */
#else
#define FLASH_CCA_BOOTLDR_CFG FLASH_CCA_BOOTLDR_CFG_DISABLE
#endif
```
and then in startup-gcc.c we'd simply assign `FLASH_CCA_BOOTLDR_CFG` to `__cca.bootldr_cfg`