devtool: correctly handle non-standard source tree locations in upgrades
When S is set to a sub-directory of upstream source, the license
checks and the bbappend writing (specifically, setting EXTERNALSRC)
need to operate on that sub-directory.
'devtool modify' already has similar logic, and it was copied from there
and adjusted.
Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com> Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
