war: Fix reference to context resource in same BUCK file

If the resource starts with ':' its within this package. Drop the
':' before passing to genfile(), without adding a '/' as a prefix.

Change-Id: I70b48433fdd95e569bc2abe89b94543641aaf639
diff --git a/war.bucklet b/war.bucklet
index 92a7acc..0394375 100644
--- a/war.bucklet
+++ b/war.bucklet
@@ -48,7 +48,9 @@
       dep.append(r[:r.rindex('.')])
       if r.startswith('//'):
         r = root + r[2:]
-      r = r.replace(':', '/')
+        r = r.replace(':', '/')
+      elif r.startswith(':'):
+        r = r[1:]
       src.append(genfile(r))
   if src:
     cmd.append('$SRCS')