Boot: Add RSA-3072 support to imgtool script

PSA TBSA-M recommends to use RSA signature for firmware
authentication with at least 3072 bits length key size.

This change is based on:
https://github.com/JuulLabs-OSS/mcuboot/pull/476
authored by Fabio Utzig <utzig@apache.org>

Change-Id: I78fb0c9732aa6942b6fcb46fef5e1965c9dccaa5
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/bl2/ext/mcuboot/scripts/imgtool_lib/image.py b/bl2/ext/mcuboot/scripts/imgtool_lib/image.py
index bd1bf5d..ff137a5 100644
--- a/bl2/ext/mcuboot/scripts/imgtool_lib/image.py
+++ b/bl2/ext/mcuboot/scripts/imgtool_lib/image.py
@@ -36,6 +36,7 @@
         'KEYHASH': 0x01,
         'SHA256' : 0x10,
         'RSA2048': 0x20,
+        'RSA3072': 0x23,
         'SEC_CNT': 0x50, }
 
 TLV_INFO_SIZE = 4