srctree.bbclass: add initial version.

srctree.bbclass: add initial version.

This enables operation inside of an existing source tree for a project, rather
than using the fetch/unpack/patch idiom.

By default, it expects that you're keeping the recipe(s) inside the
aforementioned source tree, but you could override S to point at an external
directory and place the recipes in a normal collection/overlay, if you so
chose.

It also provides some convenience python functions for assembling your
do_clean, if you want to leverage things like 'git clean' to simplify the
operation.

Signed-off-by: Chris Larson <clarson@kergoth.com>