samples: code_relocation_nocopy: Make linker script more generic

Instead of depending on the nRF5340 DK, use a path specific to just
the nRF5340 SoC and get the size of the external flash in this case
from devicetree so that the sample could be easily run on other boards
based on the nRF5340 SoC (after just adding proper overlays).

On the occasion, correct the attributes of the EXTFLASH memory region.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>