bootstd: Provide a command to select the bootdev order

It is sometimes useful to select or override the default bootdev order.
Add a command for this.

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

doc/usage/cmd/bootdev.rst

@@ -13,6 +13,7 @@ Synopsis
 
     bootdev list [-p]        - list all available bootdevs (-p to probe)
     bootdev hunt [-l|<spec>] - use hunt drivers to find bootdevs
+    bootdev order [clear] | [<spec> ...]  - view or update bootdev order
     bootdev select <bm>      - select a bootdev by name
     bootdev info [-p]        - show information about a bootdev
 
@@ -78,6 +79,27 @@ To run hunters, specify the name of the hunter to run, e.g. "mmc". If no
 name is provided, all hunters are run.
 
 
+bootdev order
+~~~~~~~~~~~~~
+
+This allows the bootdev order to be examined or set. With no argument the
+current ordering is shown, one item per line.
+
+The argument can either be 'clear' or a space-separated list of labels. Each
+label can be the name of a bootdev (e.g. "mmc1.bootdev"), a bootdev sequence
+number ("3") or a media uclass ("mmc") with an optional sequence number (mmc2).
+
+Use `bootdev order clear` to clear any ordering and use the default.
+
+By default, the ordering is defined by the `boot_targets` environment variable
+or, failing that, the bootstd node in the devicetree ("bootdev-order" property).
+If no ordering is provided, then a default one is used.
+
+Note that this command does not check that the ordering is valid. In fact the
+meaning of the ordering depends on what the bootflow iterator discovers when it
+is used. Invalid entries will result in no bootdevs being found for that entry,
+so they are effectively skipped.
+
 bootdev select
 ~~~~~~~~~~~~~~
 
@@ -171,6 +193,20 @@ This shows using one of the available hunters, then listing them::
                 Capacity: 0.0 MB = 0.0 GB (1 x 512)
     =>
 
+This shows viewing and changing the ordering::
+
+    => bootdev order
+    mmc2
+    mmc1
+    => bootdev order 'mmc usb'
+    => bootdev order
+    mmc
+    usb
+    => bootdev order clear
+    => bootdev order
+    No ordering
+    =>
+
 
 Return value
 ------------