bitbake: Correctly handle multiline comments including whitespace

If metadata contains:

"""

FOO = "bar"
"""

The variable FOO should get set to "bar" but doesn't due to the empty lines
be swallowed by the parser and FOO becomming part of the multiline comment.
This patch corrects that behaviour so FOO is set as expected.

[YOCTO #1377]

This patch fixes parsing of multiline comments so lines ending with \
behave consistently and we warn users where there is something happening
they likely don't expect.

(Bitbake rev: 30eaef7f50fff855cf8830772a7088dd83a4240e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/parse/parse_py/BBHandler.py

@@ -194,21 +194,21 @@ def feeder(lineno, s, fn, root, statements):
                 return
 
 
-    # Skip empty lines
-    if s == '':
-        return          
-
-    if s[0] == '#':
+    if s and s[0] == '#':
         if len(__residue__) != 0 and __residue__[0][0] != "#":
             bb.error("There is a comment on line %s of file %s (%s) which is in the middle of a multiline expression.\nBitbake used to ignore these but no longer does so, please fix your metadata as errors are likely as a result of this change." % (lineno, fn, s))
 
-    if s[-1] == '\\':
+    if s and s[-1] == '\\':
         __residue__.append(s[:-1])
         return
 
     s = "".join(__residue__) + s
     __residue__ = []
 
+    # Skip empty lines
+    if s == '':
+        return   
+
     # Skip comments
     if s[0] == '#':
         return