soc: stm32u5: Replace IMGTOOL_ARGS with ROM_START_OFFSET

EXTRA_IMGTOOL_ARGS is used to set additional options by the user.
Any user change will overwrite this option, which
is unintuitive.
Also option ROM_START_OFFSET will be overwritten which is also unintuitive.

Replace hardcoded config option MCUBOOT_EXTRA_IMGTOOL_ARGS
with proper config ROM_START_OFFSET.

Signed-off-by: Maciej Zagrabski <mzi@trackunit.com>