pxe: Allow calling the pxe_get logic directly

Refactor this code so that we can call the 'pxe get' command without going
through the command-line interpreter. This makes it easier to get the
information we need, without going through environment variables.

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>