devtool: error out when workspace is using old override syntax

devtool: error out when workspace is using old override syntax

When the workspace bbappends are still using the old override syntax
with EXTERNALSRC_pn-*, externalsrc_re will not match, and pn will never
be assigned, leading to a nondescript UnboundLocalError being raised on
the user's terminal. Try to detect that situation and give the user a
hint how to solve it.

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>