loader: tftp: Add preload method

The preload method will transfer the whole file in a buffer and cache it
so read/lseek operations are faster.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33410

(cherry picked from commit 3eb019000c9eba6dafad6741a88f627b4d4fcd8a)