[LEDE-DEV] Multi-Profile selection support

[Felix Fietkau]

Hi Daniel et al.

after a long delay, I finally got around to looking into multi profile
selection again. After reviewing your code and thinking about it some
more, I decided to go with a slightly different approach from the one
that you took.

I'm working towards not only being able to build an image with support
for a specific set of devices, but also being able to optionally build
multiple images at once with the per-image rootfs being affected by the
device profile as well.

This can currently only work properly with the new image building code
along with having the device profile data specified in the Device
section, so I limited multi-profile selection to that.

As a next step, I'm going to add some wrapper code to allow the legacy
image building code to be incrementally converted as well without having
to adjust everything to the new Build/* chunks right away.

You can find my work in progress in my staging tree:
https://git.lede-project.org/?p=lede/nbd/staging.git;a=summary

If you have some time, feedback and patches converting profiles/* stuff
into proper device profiles will be appreciated.

- Felix