Merge patch series "pxe: Allow extlinux booting without CMDLINE enabled"

Simon Glass <sjg@chromium.org> says:

This series is the culmanation of the current line of refactoring
series. It adjusts pxe to call the booting functionality directly
rather than going through the command-line interface.

With this is is possible to boot using the extlinux bootmeth without
the command line enabled.

It also updates fastboot to do a similar thing.