Add RSA-3072 support to imgtool

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/image.py b/scripts/imgtool/image.py
index c36f802..ad156a1 100644
--- a/scripts/imgtool/image.py
+++ b/scripts/imgtool/image.py
@@ -49,6 +49,7 @@
         'RSA2048': 0x20,
         'ECDSA224': 0x21,
         'ECDSA256': 0x22,
+        'RSA3072': 0x23,
         'ENCRSA2048': 0x30,
         'ENCKW128': 0x31,
         'DEPENDENCY': 0x40