cc2538: Clean up port and pin definitions

Homogenize port and pin definitions naming:
 - PERIPHERAL_FUNCTION_PORT for the port ID,
 - PERIPHERAL_FUNCTION_PIN for the pin ID,
 - PERIPHERAL_FUNCTION_PORT_BASE for the port base,
 - PERIPHERAL_FUNCTION_PIN_MASK for the pin mask.

Define only PERIPHERAL_FUNCTION_PORT and PERIPHERAL_FUNCTION_PIN in board.h, and
deduce PERIPHERAL_FUNCTION_PORT_BASE and PERIPHERAL_FUNCTION_PIN_MASK in the
driver from the former definitions.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: usb: Make the GPIO driving the pull-up optional

The data sheet recommends that the USB pull-up resistor be driven by a GPIO so
that it can be controlled by software, but this is not mandatory. Hence, leave
the choice so that CC253-based boards not using this option can build and work
fine.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538dk: startup-gcc: Do not redefine SYS_CTRL_EMUOVR

SYS_CTRL_EMUOVR is already defined in sys-ctrl.h, so #include this header file
instead of redefining SYS_CTRL_EMUOVR in startup-gcc.c.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
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>
Remove LPP discussion from the cc2538 README
Merge pull request #463 from ADVANSEE/cc2538-ports-pins

cc2538: Clean up and improve port and pin definitions
Merge pull request #491 from g-oikonomou/remove-lpp-from-readme

Remove LPP discussion from the cc2538 README
Merge pull request #478 from ADVANSEE/cc2538-startup-cleanup

cc2538: Startup cleanup