cc2538: Add header file for flash CCA page and use it

Create a dedicated header file with all the definitions for the flash lock bit
page and customer configuration area. This avoids duplicating those definitions
in the startup-gcc.c files of all CC2538-based platforms, and this also allows
to easily manipulate the CCA from outside startup-gcc.c (e.g. for on-the-air
firmware update).

The definitions are now complete contrary to what was in startup-gcc.c:
 - Definitions have been added to select the bootloader backdoor pin and active
   level if enabled.
 - Definitions have been added to access the page and debug lock bits. The debug
   lock bit can be used to prevent someone from reading back a programmed
   firmware through JTAG if the firmware binary image has to be confidential,
   which should be combined with a disabled bootloader backdoor.
 - The application entry point is now tied to the beginning of the .text section
   instead of to the beginning of the flash. This allows projects using custom
   linker scripts to place the application entry point anywhere in the flash,
   which can be useful e.g. for on-the-air firmware update.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>