devtool: ignore pn- overrides when determining SRC_URI overrides

If (perhaps foolishly) at your configuration level you have e.g.

  SRC_URI_append_pn-recipename = " file://patchname.patch"

and then run devtool modify on a different recipe, an error occurs:

  INFO: SRC_URI contains some conditional appends/prepends - will create branches to represent these
  ...
  ERROR: [Errno 2] No such file or directory: '/path/to/downloads/patchname.patch'

pn- overrides would not constitute an alternative configuration that we
should handle in this context, so just ignore them to avoid the issue.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3f2a812a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>