kconfig: mcuboot: Add MCUBOOT_IMGTOOL_OVERWRITE_ONLY option

Add MCUBOOT_IMGTOOL_OVERWRITE_ONLY Kconfig option which
passes the --overwrite-only option to imgtool to avoid
adding the swap status area size when calculating overflow.
It is used by non-swap update modes.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>