Major changes including device architecture detection

This MR introduces (breaking) changes:

• adds architecture detection on connected device
• make 20.04 focal the default framework (instead of 16.04 xenial)
• commands are classified as device, arch-specific and/or build commands
  • --ssh and --serial-number are only available for device commands
  • makes architecture detection the default for device commands
  • opt-in architecture detection for all arch-specific commands via --arch detect or always_detect in the global Clickable config
  • device commands allow to specify a target with --target adb|ssh|host|detect, where the host target facilitates the use of Clickable running directly in an UT environment
• default_target configurable in the global Clickable config
• default_arch in the build section of the global Clickable config allows to build build for a specific architecture by default (when device architecture detection is not active)
• suppress SSH welcome message by touching /home/phablet/.hushlogin
• update docs, including Clickable 8 Migration guide

Deprecation/Removal:

• arch field in the device section is ignored, because the device architecture is detected automatically. Note that for build commands, a default architecture can be set as default_arch.
• drop Cordova support and remove nodejs from images (solves #407 (closed))