The RSA signature code in mcuboot is based on PKCS#1 v1.5, which uses a fixed padding. The latest version of this spec PKCS#1 v2.1/RFC3447 recommends that new applications be designed around a newer RSA-PSS padding. Although there are no known weaknesses at this time in the v1.5 padding, the PSS padding has a security proof.
The change in mcuboot itself is likely a matter of configuring the build of mbedTLS. Both Go and pycrypto support RSA-PSS, so changes to these tools to support signing with RSA-PSS are also small changes.
This should start with an analysis of the code size changes, and the signature performance.