fnginstall: add postinstallation step and use it for kernel/dtb blob creation

* Remove the kernel/dtb creation and platform detection from fnginstall
* Added a target_os_postinstallation step to execute an command
  (specified by dedicated name) inside the target os via change root
  ** like user script
  ** only if the command is available
  ** this way the installation script stays generic
* Added a post installation command to the image to detect the platform
  and create the kernel/dtb blob, this should later also integrate the
  config via xml to devicetree stuff
* Moved fnginstall and fnginstall-postinstallation to
  packagegroup-core-boot using the 'fng' machine feature: This way
  the fng stuff lives in the distro layer but is enabled machine
  dependend, without adding direct dependency between the layers