bootstd: Add an implementation of distro boot

Add a bootmeth driver which handles distro boot from a disk, so we can
boot a bootflow using this commonly used mechanism.

In effect, this provides the same functionality as the 'sysboot' command
and shares the same code. But the interface into it is via a bootmeth.

For now this requires the 'pxe' command be enabled. Future work may tidy
this up so that it can be used without CONFIG_CMDLINE being enabled.

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