cmd: pxe_utils: sysboot: add label override support

This will allow consumers to choose a pxe label at runtime instead of
having to prompt the user. One good use-case for this, is choosing
whether or not to apply a dtbo depending on the hardware configuration.
e.g: for TI's AM335x EVM, it would be convenient to apply a particular
dtbo only when the J9 jumper is on PRUSS mode. To achieve this, the
pxe menu should have 2 labels, one with the dtbo and the other without,
then the "pxe_label_override" env variable should point to the label with
the dtbo at runtime only when the jumper is on PRUSS mode.

This change can be used for different use-cases and bring more
flexibilty to consumers who use sysboot/pxe_utils.

if "pxe_label_override" is set but does not exist in the pxe menu,
the code should fallback to the default label if given, and no failure
is returned but rather a warning message.

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