scripts: assemble: Fix problem with missing output

Adding an os.unlink() call to remove the outfile results in an exception
being thrown if the file does not exist.  Fix this by trapping, and
checking for the specific error we get on a missing file.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/assemble.py b/scripts/assemble.py
index 92623c4..30adcba 100755
--- a/scripts/assemble.py
+++ b/scripts/assemble.py
@@ -5,6 +5,7 @@
 """
 
 import argparse
+import errno
 import io
 import re
 import os.path
@@ -25,7 +26,11 @@
 class Assembly():
     def __init__(self, output, bootdir):
         self.find_slots(bootdir)
-        os.unlink(output)
+        try:
+            os.unlink(output)
+        except OSError as e:
+            if e.errno != errno.ENOENT:
+                raise
         self.output = output
 
     def find_slots(self, bootdir):