RFC - Multiple Architecture on one Image

Overview

The idea is to implement support for having different architecture of the same distribution on one live image.

defaults.sh needs to parse config/foo first, then config/foo.$arch second.
lb config needs to understand architectures as a seperate context:
- 'lb config -a amd64' will apply on config/*.amd64 files.
- 'lb config -a i386' will apply on config/*.i386 files.
- 'lb config -a any' will apply on config/* files (default).
lb build needs to understand architectures as a seperate context:
- 'lb build -a amd64' will only build the amd64 images.
- 'lb build -a all' will build all possible architecture images (default).

Use filesystem.list to specify which filesystems need to be mounted in which order.

Needs to mount and copy any architecture independent filesystem image (filesystem.all.squashfs) first.
Needs to mount and copy any architecture dependet filesystem image (filesystem.i386.squashfs) second.
If lacking any of above, look normally for a single filesystem image (filesystem.squashfs).

Needs to prepared to incorporate additional menu entries.
Needs to autodetect the default architecture (ifcpu64.c32).
If using gfxboot, there should be an architecture switch to choose the architecture from.