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