nrfjprog.py: Fail if hex file has UICR data and no --erase

Inspect the hex file with intelhex, and fail if the hex file has any
contents in the UICR area(s).
family == 'NRF52' still always does --sectoranduicrerase, but this
option is not available on other families.
Add --force command line option to proceed with flashing instead of
failing.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>