bitbake: bb.cooker: show limited traceback for parsing ExpansionError

It's useful to see tracebacks for ExpansionErrors, but only if we skip the
leading bitbake-internal elements, otherwise we see elements of the expansion
process.

As one example:

Before:

    ERROR: ExpansionError during parsing /scratch/yocto-new/external-as-needed/poky/meta/recipes-core/glibc/glibc-locale_2.23.bb: Failure expanding variable PV[:=], expression was ${@get_external_libc_version(d)} which triggered exception AttributeError: 'module' object has no attribute 'external'

After:

    ERROR: ExpansionError during parsing /scratch/yocto-new/external-as-needed/poky/meta/recipes-core/glibc/glibc-locale_2.23.bb
    Traceback (most recent call last):
      File "PV[:=]", line 1, in <module>
      File "/scratch/yocto-new/external-as-needed/meta-sourcery/recipes-external/glibc/glibc-external-version.inc", line 3, in get_external_libc_version(d=<bb.data_smart.DataSmart
    object at 0x7f05d2566950>):
             sopattern = os.path.join(d.getVar('base_libdir', True), 'libc-*.so')
        >    found_paths = oe.external.find_sysroot_files([sopattern], d)
             if found_paths:
    ExpansionError: Failure expanding variable PV[:=], expression was ${@get_external_libc_version(d)} which triggered exception AttributeError: 'module' object has no attribute 'external'

(Bitbake rev: 7ff5b9eed82b7f4fd138fc6d746a0b79efbea98a)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cooker.py

@@ -2147,8 +2147,11 @@ class CookerParser(object):
             return False
         except bb.data_smart.ExpansionError as exc:
             self.error += 1
-            _, value, _ = sys.exc_info()
-            logger.error('ExpansionError during parsing %s: %s', value.recipe, str(exc))
+            bbdir = os.path.dirname(__file__) + os.sep
+            etype, value, _ = sys.exc_info()
+            tb = list(itertools.dropwhile(lambda e: e.filename.startswith(bbdir), exc.traceback))
+            logger.error('ExpansionError during parsing %s', value.recipe,
+                         exc_info=(etype, value, tb))
             self.shutdown(clean=False)
             return False
         except Exception as exc: