Draft: run fng postinstallation on rauc post install

This is a fix for issue with ID [746-000703] RAUC: Adapt post-install to RAUC.

Initial version, just to see how it will look for Edgehog builds.

Except for addressing the problem articulated by SECO in 746-000703 (calling fnginstall-postinstall), this MR is also intended to check whether SECO NE layers are compatible with RAUC and what should be done to achieve this goal. Herewith we've made following assumptions:

• Edgehog is already compatible with RAUC;
  
• our main goal would be not to reinvent the wheel again (integrate RAUC into SECO NE layers directly) but to adjust RAUC in Edgehog (and Edgehog itself) to be compatible with SECO NE boards/layers.

And during this work, we found that following problems should be still resolved to make SECO NE layers RAUC ready:

• (Edgehog) bootfs_x partitions are not included in the "seco-bundle-edgehog" and won't be touched during update;
• (Edgehog) /etc is on separate partition in Edgehog, so files on it won't be touched by RAUC during update. Herewith RAUC scripts and configs are located on /etc itself;
• (Edgehog) we can't imagine how to add post installation scripts for RAUC without using dynamic layers, which is not used by SECO at the moment.

Known limitations of current implementation of fnginstall-postinstall:

• /etc/fng-postinstall/30-prepare-bootscript, which is called by fnginstall-postinstall out of box, will fail and cause "post-installation" to fail as well. So, we just disabled it for now.
• moreover, please be aware that bootargs are not updated by 30-prepare-bootscript at all;
• we hardcoded names of partitions to handle during post installation in the script itself. It would be better to extract them from system.conf for RAUC.