Fix bitfield prefix stripping for indexed peripherals

[MabezDev]

This also reduces the compile time, as copy's meant duplicate codegen, derive uses a single source for X peripherals.

Unfortunately, I've had to remove the interrupts for the indexed peripherals atm. Including _indexed_interrupts.yaml causes svdpatch to die:

svd patch svd/patches/esp32.yaml
Traceback (most recent call last):
  File "/home/mabez/.local/bin/svd", line 8, in <module>
    sys.exit(svdtools_cli())
  File "/home/mabez/.local/lib64/python3.6/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/mabez/.local/lib64/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/mabez/.local/lib64/python3.6/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/mabez/.local/lib64/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/mabez/.local/lib64/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/mabez/.local/lib64/python3.6/site-packages/svdtools/cli.py", line 16, in patch
    svdtools.patch.main(yaml_file)
  File "/home/mabez/.local/lib64/python3.6/site-packages/svdtools/patch.py", line 1004, in main
    process_device(svd, root)
  File "/home/mabez/.local/lib64/python3.6/site-packages/svdtools/patch.py", line 984, in process_device
    d.process_peripheral(periphspec, device[periphspec], update_fields)
  File "/home/mabez/.local/lib64/python3.6/site-packages/svdtools/patch.py", line 427, in process_peripheral
    raise MissingPeripheralError("Could not find {}".format(pspec))
svdtools.patch.MissingPeripheralError: Could not find UHCI0
make: *** [Makefile:11: patch] Error 1

Quite a strange error as UHCI0 and the likes are actually already in the base svd file, so I don't really understand how it's not finding it.

Closes #17

[davidkern]

Unfortunately that was the same issue I ran into adding the interrupts. Switching them over to copy instead was the only thing I could figure out to get it to work (but then broke this instead).

I'm still hoping to fix this upstream in the idf2svd tool, but haven't had time to work on this recently.

[MabezDev]

I think I've fixed the issue now (provided CI passes), but you're right I think this would be better if it could be derived with idf2svd. One quite nice benefit of derive is that it reduced the compile time massively.