pxeboot: Add nfs.read_size tunable.

Increase this tunable improves kernel loading speed.

Submitted by: Jun Su <junsu microsoft com>
Reviewed by: rpokala, wblock (previous version)
Obtained from: DragonFlyBSD
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7756