gen-device: only generate device files for supported devices

[niaow]

This changes the make gen-device command such that it only converts relevant device files. This saves a lot of build time.

The process of determining which files to convert is now handled by make. Incremental builds are now supported.
Appropriate parallel building is now done for all device generation (previously it was only handled properly for AVR).

The generated file names were updated to match their source name formatting. I could probbably lowercase everything when converting them, but this would complicate the makefile. I don't really think this is worth it.

[niaow]

Before:

real    0m48.662s
user    6m5.821s
sys     0m42.683s
539505021 total

After:

real    0m1.934s
user    0m26.767s
sys     0m3.649s
77640936 total

This is much faster and it reduces the storage used by device files by 86%.

[b0ch3nski]

Might not be in scope of this actual PR but I stumbled across this some time ago - this regex is unnecessarily compiled multiple times

[dgryski]

Seems reasonable, although I'll leave it to someone who actually deals with this files for the final word.

[deadprogram]

The generated file names were updated to match their source name formatting. I could probbably lowercase everything when converting them, but this would complicate the makefile. I don't really think this is worth it.

I think we should probably preserve the lowercase file names if we can to avoid case issues on OS like Windows.

[niaow]

The generated file names were updated to match their source name formatting. I could probbably lowercase everything when converting them, but this would complicate the makefile. I don't really think this is worth it.

I think we should probably preserve the lowercase file names if we can to avoid case issues on OS like Windows.

I am not entirely sure what you mean by this. I understand that Windows is case-insensitive, but we are just matching the source case here?

[niaow]

I made a patch that changes them back to lowercase. It is much more complicated in the makefile since we cannot just use pattern rules (which expect the source and destination name stems to match). I had to shell out to tr to change the case and use eval to construct rules programmatically. I left this change in a separate commit so we can roll this back easily if is not worthwhile.

[deadprogram]

I was testing a little and now I see what you meant. I also am not sure if the case thing is worth it, since all of the real work happens due to either build tags or the target JSON file anyhow.

Good that you made it a separate commit, let me consider my opinion now.

BTW the speed of it is fantastic.

We will also have to think about how to modify the instructions for adding a new board, since you will have to add to the makefile to get it generated.