x86: coreboot: Update doc for CBFS access

Add an example to show how cbfs is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[Removed CONFIG_CMD_CBFS from defconfig files]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

doc/board/coreboot/coreboot.rst

@@ -51,6 +51,40 @@ can be useful for running UEFI applications, for example.
 
 This has only been lightly tested.
 
+CBFS access
+-----------
+
+You can use the 'cbfs' commands to access the Coreboot filesystem::
+
+   => cbfsinit
+   => cbfsinfo
+
+   CBFS version: 0x31313132
+   ROM size: 0x100000
+   Boot block size: 0x4
+   CBFS size: 0xffdfc
+   Alignment: 64
+   Offset: 0x200
+
+   => cbfsls
+        size              type  name
+   ------------------------------------------
+          32       cbfs header  cbfs master header
+       16720                17  fallback/romstage
+       53052                17  fallback/ramstage
+         398               raw  config
+         715               raw  revision
+         117               raw  build_info
+        4044               raw  fallback/dsdt.aml
+         640       cmos layout  cmos_layout.bin
+       17804                17  fallback/postcar
+      335797           payload  fallback/payload
+      607000              null  (empty)
+       10752         bootblock  bootblock
+
+   12 file(s)
+
+   =>
 
 Memory map
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