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>

doc/develop/bootstd/overview.rst

@@ -103,6 +103,12 @@ provide a `read_bootflow()` method which checks whatever bootdevs it likes, then
 returns the bootflow, if found. Some of these bootmeths may be very slow, if
 they scan a lot of devices.
 
+The extlinux bootmeth also allows for bootmeth specific configuration to be
+set. A bootmeth that wishes to support this provides the `set_property()`
+method. This allows string properties and values to be passed to the bootmeth.
+It is up to the bootmeth to determine what action to take when this method is
+called.
+
 
 Boot process
 ------------
@@ -459,8 +465,8 @@ Three commands are available:
     See :doc:`/usage/cmd/bootflow`
 
 `bootmeth`
-    Allow listing of available bootmethds and setting the order in which they
-    are tried. See :doc:`/usage/cmd/bootmeth`
+    Allow listing of available bootmethds, setting the order in which they are
+    tried and bootmeth specific configuration. See :doc:`/usage/cmd/bootmeth`
 
 .. _BootflowStates: