Option to generate i386-compatible images

[pablobm]

We recently received a new laptop (a donation) with an architecture not compatible with amd64 builds. Fortunately, we can create compatible images with some small changes.

By default, the makefile will continue generating amd64 images. We can generate i386 images by running make i386.

Initially, this should be as simple as mentioning the desired architecture compatibility when calling lb config. Unfortunately there were a few hitches:

• If we generate an amd64 image, then a i386 image, the process will fail. Deleting the cache directory before a build solves this, but deleting the cache slows the build significantly. As a solution, I'm keeping architecture-specific cache directories, symlinked from cache. The Live Build commands do not appear to allow us to customize the cache directory, hence the symlink.
• The makefile is now recursive to work around limitations regarding variables (too complex to explain here, or to remember for that matter).
• The image should be i386-compatible, but the Thonny image is for i686, so don't expect to run this in anything less than a Pentium. (Yes, I know that would be the least of your problems if you were running this on a 486 machine with 8MB of RAM, or whatever your IoT-enabled pedal bin runs).