[oe] Change ANGSTROM_EXTRA_INSTALL to IMAGE_EXTRA_INSTALL

[Rolf Leggewie]

Marcin Juszkiewicz wrote:
> I vote against IMAGE_EXTRA_INSTALL. Let it live as ugly Angstrom hack for 
> breaking sane images into unsupportable ones.

How would the correct way to do this look like?

I regularly want to include package X for testing it out.  I don't want
to create a new image definition every time.