bootstd: Add command to enable setting of bootmeth specific properties

bootstd: Add command to enable setting of bootmeth specific properties

We have previously added logic to allow a "fallback" option to be
specified in the extlinux configuration. Provide a command that allows
us to set this as the preferred default option when booting.

Combined with the bootcount functionality, this allows the "altbootcmd"
to provide a means of falling back to a previously known good state
after a failed update. For example, if "bootcmd" is set to:

    bootflow scan -lb

We would set "altbootcmd" to:

    bootmeth set extlinux fallback 1; bootflow scan -lb

Causing the boot process to boot from the fallback option.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Martyn Welch <martyn.welch@collabora.com>