Add the target config option "boot-stack-size" which is passed to the
linker as the define "MBED_BOOT_STACK_SIZE" so the linker can
adjust the stack accordingly. On mbed 2 the boot stack becomes the
main stack after boot.  On mbed 5 the boot stack becomes the
ISR stack after boot. Because of these different uses  the stack size
for mbed 2 is set to 4K by default while on mbed 5 it is set to 1k.

Additionally, the NRF5X family requires a larger interrupt stack size
due to the softdevice so the size  is increased to 2k on mbed 5 builds.