libpayload: Remove file I/O in flashrom.c.

read_buf_from_file() and write_buf_to_file() use file streams which are
not supported in libpayload.

Signed-off-by: Nico Huber <nico.huber@secunet.com>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1730 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1