devtool: process local files only for the main branch

devtool modify/upgrade are not currently equipped to handle conditional local files
in SRC_URI, and provide only the main no-override set in a workspace under
source/component/oe-local-files/ (this is done via meta/classes/devtool-source.bbclass).

On the other hand, updating the changes from workspace into a recipe
is run iteratively against all overrides; this works for patches (as they
all are directed into their own override branches in the workspace
git source tree), but breaks down when trying to match local files
in a workspace against local files in overridden SRC_URI lists, resulting in
bad recipe breakage.

(there's an additional twist here: existing code has a guard against this
but the guard relies on metadata in workspace .bbappend that is only there
in modify operations, but not upgrades. This commit replaces the guard
with a general check that will work everywhere).

Implementing multiple sets of local files is significant work; let's for now
simply not touch local files in recipes except when on the no-override variant.

Also, adjust the selftest cases to include conditional local files in sample
recipes, so the situation is covered by the tests.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>