assemble: Assemble without image-1 partition

[toshbi4]

Motivation

It is impossible to use assemble script with SINGLE_APPLICATION_SLOT application configuration.
https://github.com/mcu-tools/mcuboot/blob/main/boot/zephyr/Kconfig#L62

Testing

Tested on zephyr application with CONFIG_SINGLE_APPLICATION_SLOT enabled and mcuboot + image-0 partition in .dts provided.

Changes

Now image-1 absence exception is triggered only if secondary firmware path was provided with -s argument to the script.