Why would you want to encrypt /boot?
Just keep it unencrypted. There is nothing in this directory (partition) that you'd need to keep private in any scenario.
It's standard practice for full-disk encryption on Linux to have a fully encrypted LVM plus an unencrypted boot partition. If an attacker is in a position to tamper with the contents of /boot, they already have direct access to your computer. Meaning that you can pretty much consider the device compromised anyway (if that's your paranoia mode). There's nothing an OS can do to help in this scenario. If that's your threat profile, you'll have to take additional (physical) steps to secure your data. If not, then just keep /boot as it is.