We need an SDK

Notes by @butirsky:

• I think it's good idea to provide the link to @hanyoung's blog about PlaMo cross-compiling for Manjaro right here, because it seems to be the only source of a meaningful info for the moment, combined together:
  https://www.hanyoung.uk/blog/cross-compile-to-pinephone-part-one
  Thanks Han!
• the current cross-compiling approach with the SDK is documented here:
  https://community.kde.org/Plasma/Mobile/DevGuide#General_Linux-based_development_environment
  You are welcome to try it.