boot/boot_serial: build-time skip of the erasing of img status page

For MCUboot's MCUBOOT_SINGLE_APPLICATION_SLOT, MCUBOOT_FIRMWARE_LOADER,
MCUBOOT_SINGLE_APPLICATION_SLOT_RAM_LOAD
modes there is not image run-time status data at the end on the image.
Erase of that was done as the last image page erase - and was no harm
before by luck.

Following commit just reveled the issue:
 #35536633

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>