bootstd: Treat DHCP and PXE as bootdev labels

These are associated with the ethernet boot device but do not match its
uclass name, so handle them as special cases.

Provide a way to pass flags through with the bootdev so that we know
how to process it. The flags are checked by the bootmeths, to ensure that
only the selected bootmeth is used.

While these both use the network device, they work quite differently. It
is common to run only one of these, or to run PXE before DHCP. Provide
bootflow flags to control which methods are used. Check these in the two
bootmeths so that only the chosen one is used.

Signed-off-by: Simon Glass <sjg@chromium.org>