Boot: Add RSA-3072 support to MCUBoot

PSA TBSA-M recommends to use RSA signature for firmware
authentication with at least 3072 bits length key size.
This patch introduces:
- add an example RSA-3072 key
- add configurable RSA-3072 support (RSA-2048 still available)
- set RSA-3072 to default

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

Change-Id: Ic8d188f64d0dbe54aebf28c2778fb932e1afeeb9
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/CommonConfig.cmake b/CommonConfig.cmake
index c75cc79..668906b 100644
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -230,7 +230,7 @@
 		message(FATAL_ERROR "MCUBoot only supports RSA-2048 and RSA-3072 signature")
 	endif()
 	if (NOT DEFINED MCUBOOT_SIGNATURE_TYPE)
-		set(MCUBOOT_SIGNATURE_TYPE "RSA-2048")
+		set(MCUBOOT_SIGNATURE_TYPE "RSA-3072")
 	endif()
 	if (NOT ${MCUBOOT_UPGRADE_STRATEGY} STREQUAL "OVERWRITE_ONLY" AND
 		NOT ${MCUBOOT_UPGRADE_STRATEGY} STREQUAL "SWAP" AND