initial flex support

[adamcstephens]

I'm mostly just following the existing pattern. The codegen mod.rs file is a copy of 2x4hd, which may or may not match all the options of the flex. If you have any pointers on how to verify the input/output options, please let me know.

If I missed anything or did anything wrong, please point it out.

resolves: #370

[mrene]

Thanks!

Codegen at least verifies that there are no missing symbols since it has to look them up from the xml config. Since Flex shares the same app as the 2x4hd I assume it'd be sufficient.

[Mobe1969]

I think the build might be broken for the pi version, armhf.deb

When I run minidsp:

minidsp: /lib/arm-linux-gnueabihf/[libc.so](http://libc.so/).6: version `GLIBC_2.33' not found (required by minidsp) 
minidsp: /lib/arm-linux-gnueabihf/[libc.so](http://libc.so/).6: version `GLIBC_2.32' not found (required by minidsp)

I've been testing on my windows test rig and works well on that.