west: runner: Use --verify and --verify-only with openocd and hex files

the options --verify and --verify-only where only used when flashing elf
files and were ignored for flashing hex files for example when using
openocd runner.
The command to verify the image is now dependant on the presence of
--verify or --verify-only, and the load command is not executed if
--verify-only is provided.

Signed-off-by: Nicolas VINCENT <nicolas.vincent@vossloh.com>