pxe: Move pxe_utils files

Move the header file into the main include/ directory so we can use it
from the bootmethod code. Move the C file into boot/ since it relates to
booting.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Tested-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>