devtool: modify: make bitbake use local files from srctree

devtool: modify: make bitbake use local files from srctree

This change makes it possible to have local files (non-remote SRC_URI
files, i.e. files that are located in the "recipe space") under the
srctree even if S!=WORKDIR. The files must be placed under the
'local-files' subdirectory.

Complements the previous patch that imports local files into srctree.

[YOCTO #7602]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>