wic: fix pylint warning unused-variable

Removed or reworked code with unused variables.

(From OE-Core rev: 3644b9f5ca25dfc61d4f5eda0f073c8a879b492b)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/engine.py

@@ -43,9 +43,7 @@ def verify_build_env():
 
     Returns True if it is, false otherwise
     """
-    try:
-        builddir = os.environ["BUILDDIR"]
-    except KeyError:
+    if not os.environ.get("BUILDDIR"):
         print "BUILDDIR not found, exiting. (Did you forget to source oe-init-build-env?)"
         sys.exit(1)