[nrfconnect] Refactored configuration for child images

[kkasperczyk-no]

Problem

Currently used approach for nrfconnect regarding child images results in significant duplication of configuration files,
what hardens maintenance.

Change overview

• Created Kconfig.mcuboot.defaults file that sets default configuration for all examples using mcuboot child image
• Created Kconfig.multiprotocol_rpmsg file that sets default configuration for all examples using multiprotocol
  rpmsg child image
• Removed from rpc.overlay comment describing how to use it, as it duplicates example README.md information

[Damian-Nordic]

Does the change mean that we're able to flatten the configuration tree and not to duplicate files across boards?

[kkasperczyk-no]

Does the change mean that we're able to flatten the configuration tree and not to duplicate files across boards?

A little bit. We still need to duplicate prj.confs for mcuboot and multiprotocol_rpmsg in the child_image directories, as not all configs can be set in defaults, due to overrides in board files. At least we will get rid of plenty overlays containing the same options and having different names (like multiprotocol_rpmsg, multiprotocol_rpmsg_release, multiprotocol_rpmsg_no_dfu)

[Damian-Nordic]

Looks good. Please verify .config files (in both app/net/mcuboot images) for at least one example before and after your change :)

[kkasperczyk-no]

Please verify .config files (in both app/net/mcuboot images) for at least one example before and after your change :)

I tried to do so, but it's not so easy, as order of configs was changed because of using the defaults, so diff didn't help and I had to do this manually.