nrf/boards: Update memory.ld to include bootloader offsets.
nrf/boards: Update memory.ld to include bootloader offsets.

Adding variables that can be set from other linker scripts:

- _bootloader_head_size:
    Bootloader flash offset in front of the application.

- _bootloader_tail_size:
    Bootloader offset from the tail of the flash.
    In case the bootloader is located at the end.

- _bootloader_head_ram_size:
    Bootloader RAM usage in front of the application.

Updated calculations of application flash and RAM.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython