[poky] questions about bblayers idioms

[Angus Lees]

I've been trying to play nice and describe my poky changes in
self-contained layers.
I have accumulated a bunch of questions about some common problems
I've hit, and I'm sorry if some of these are FAQs:

1. *.bbappend lets me modify *.bb files.  How do I incrementally
modify other files?

Eg: I'd really like to add something to gcc-common.inc (for example).
Do I need to add a family of gcc-*.bbappend files to hook in the
common change?

1a. How do I modify/fix a bbclass file?

In particular {siteinfo,insane}.bbclass need to be modified to add new
architectures, which seems like a common task.
Copying and modifying (as I have done), gets increasingly ugly now
that I have two layers, which each want to add new architectures (one
SDK, one target).

2. BBCLASSEXTEND doesn't work in .bbappend files.

Is this correct?  I tried to add BBCLASSEXTEND+="nativesdk" to a
.bbappend file but it didn't seem to have any effect.

3. What is the standard idiom for picking up patches from layers?

I've been using something like this:
 THISDIR := "${@os.path.dirname(bb.data.getVar('FILE', d, True))}"
 FILESPATH =. "${@base_set_filespath(["${THISDIR}/${BP}"], d)}:"

But this gets increasingly more difficult once you have multiple
layers and I had to use unique names for THISDIR in each layer :(
Is there some better way to handle this?

 - Gus