Fix GELIBoot support for GELI sector size is > 512

Add support for 4k sector GELI encrypted partitions to the bootloader
This is the default created by the installer

Because the IV is different for each sector, and the XTS tweak carries forward you can not decrypt a partial sector if the starting offset is not 0

Make boot2 and the loader read in 4k aligned chunks

Reviewed by: ed, oshogbo
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D5820