pxe: Tidy up the is_pxe global

Move this into the context to avoid a global variable. Also rename it
since the current name does not explain what it actually affects.

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>