arm: Modify linker script to accomodate need for flash footer.

The porting of the TI CC2650 SoC introduces the need to
write a specific configuration area (CCFG) at the end of the
flash. It is read by the bootloader ROM of the SoC.

For now, this is a quick hack and not a generic solution;
similar needs may arise with other hardware.

Signed-off-by: Geoffrey Le Gourriérec <geoffrey.legourrierec@smile.fr>