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>
3 files changed