pxe: Return the file size from the getfile() function

It is pretty strange that the pxe code uses the 'filesize' environment
variable find the size of a file it has just read.

Partly this is because it uses the command-line interpreter to parse its
request to load the file.

As a first step towards unwinding this, return it directly from the
getfile() function. This makes the code a bit longer, for now, but will be
cleaned up in future patches.

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>