cmd: xxd: add new command

Add xxd command to print file content as hexdump to standard out

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Roger Knecht <rknecht@pm.me>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index a05aa42..64a2658 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -72,6 +72,7 @@
    cmd/true
    cmd/ums
    cmd/wdt
+   cmd/xxd
 
 Booting OS
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