boards: arm: nrf52840_pca10059: correctly set FLASH_LOAD_OFFSET

Set FLASH_LOAD_OFFSET correctly (accounting for Nordic MBR) when
BOARD_HAS_NRF5_BOOTLOADER is defined and we're not compiling MCUboot.
MCUboot will select USE_CODE_PARTITION, which will make it link
correctly regardless of which board DTS is used (stock/debugger).

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>