devtool: extract: prevent externalsrc from interfering with extraction

In case the user has set up externalsrc outside of devtool, force
EXTERNALSRC to blank for the recipe when extracting so that the original
source URI is still in SRC_URI and we're still able to extract it. (This
isn't a problem with devtool itself because the bbappends within the
workspace layer that apply externalsrc are explicitly filtered out when
devtool parses a recipe).

(From OE-Core rev: 5be16d639d1b78d114755bfd552ac901d0fdf4a9)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/devtool/standard.py

@@ -294,6 +294,8 @@ def _extract_source(srctree, keep_temp, devbranch, d):
 
         task_executor = BbTaskExecutor(crd)
 
+        crd.setVar('EXTERNALSRC_forcevariable', '')
+
         logger.info('Fetching %s...' % pn)
         task_executor.exec_func('do_fetch', False)
         logger.info('Unpacking...')