Linux for many years as supported encrypting most partitions on your system, with the exception of /boot.
/boot contains the basic/initial BOOT configuration of your system... that means, by definition, it must be discernable---and thus cannot be encrypted. Without an un-encrypted /boot partition, there isn't sufficient intelligence for the physical computer to get booted up.
From: Dave Johansen <davejohansen@gmail.com>
To: Community support for Fedora users <users@lists.fedoraproject.org>
Sent: Friday, July 31, 2015 11:28 AM
Subject: /boot and encrypted partitions?