lib/oe/classextend: Ensure we don't extend expressions more than once

We could end up with MLPREFIX being prepended to variables like
PACKAGE_DYNAMIC. This patch avoids the problem and unbreaks builds.

[YOCTO #3389]

(From OE-Core rev: 5e4714a454f9f742bf8af70dad2aa66ccb87e3fd)

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

meta/lib/oe/classextend.py

@@ -40,7 +40,9 @@ class ClassExtender(object):
         var = var.split()
         newvar = []
         for v in var:
-            if v.startswith("^"):
+            if v.startswith("^" + self.extname):
+                newvar.append(v)
+            elif v.startswith("^"):
                 newvar.append("^" + self.extname + "-" + v[1:])
             else:
                 newvar.append(self.extend_name(v))