sanitycheck: filter out the prebuilt.elf introduced by Makefile cleanup

commit 018b831a369d6b3435bd1d6ecfe7b77915dd0acf keeps another elf file in the
outdir which confused the sanitycheck script.

Change-Id: I87a9bcee6afe6526ecc24f1f5d9cf38101247ce2
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/scripts/sanitycheck b/scripts/sanitycheck
index ff96eeb..fe70926 100755
--- a/scripts/sanitycheck
+++ b/scripts/sanitycheck
@@ -1194,6 +1194,7 @@
         @return A SizeCalculator object
         """
         fns = glob.glob(os.path.join(self.outdir, "*.elf"))
+        fns = [x for x in fns if not x.endswith('_prebuilt.elf')]
         if (len(fns) != 1):
             raise BuildError("Missing/multiple output ELF binary")