pmbr: Only load the first 545k rather than error out

It would be nice to have larger boot partitions for ESPs to live in one
day. It's trivial to carve out 5M 10M or 200M when provisioning, but
logistical issues may make it hard to do it after the fact. So only warn
when the partition is > 545k. If we ever grow the boot loader larger
than that, then it will be responsible for loading the rest anyway.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D42774

(cherry picked from commit c596126a5d3d2ee015ee6807b4041efa5b9d9b07)