Handle that `IMAGE_OFFSET` can be set to something that contain spaces.
With 'IMAGE_OFFSET=$$((0x1000 + 0x1000))' we had in the command line when doing
'flash'
--offset $((0x1000 --offset + --offset 0x1000))
With the change we correctly have
--offset $((0x1000 + 0x1000))
Allow flashing with an offset in ROM from the rom base address.
It reuses `IMAGE_OFFSET` configuration variable name from `openocd.sh`.
This will allow flashing multiple images with different flash operations.
