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

commit 018b831a36 keeps another elf file in the
outdir which confused the sanitycheck script.

Change-Id: I87a9bcee6afe6526ecc24f1f5d9cf38101247ce2
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

scripts/sanitycheck

@@ -1194,6 +1194,7 @@ class TestInstance:
         @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")