remove single thread support

[nashif]

Single thread support was added to reduce the footprint for use in mcuboot which did not require any multithreading or synchronisation.
Meanwhile we are adding subsystems and drivers into mcuboot that depend on multithreading features that makes single-thread support redundant.

Reduce complexity and provide just multi-threading support and focus on reducing footprint for all users.

[nvlsianpu]

Meanwhile we are adding subsystems and drivers into mcuboot that depend on multithreading features

@nashif Which one? Recently with @carlescufi we eliminated all multithreading dependencies in mcuboot@zephyr - maybe I don't know about something.

cc @utzig @d3zd3z

[Olivier-ProGlove]

Zephyr MCUBoot variant relies on Zephyr drivers. There is no guarantee Zephyr drivers do not use Zephyr kernel API that relies on multithreading support. see my pull-request for the discussion: #8368

We have three solutions:

• Use MULTITHREADING=y for MCUBoot Zephyr's backend - current direction followed by @nashif
• Disable Zephyr kernel API that are not suitable for MULTITHREADING=n
• Add a big warning next to MCUBoot Zephyr's backend to leave the developer to carefully review the Zephyr drivers, he/she uses.