cmd: cat: add new command

Add cat command to print file content 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 d064110..a05aa42 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -31,6 +31,7 @@
    cmd/bootmeth
    cmd/button
    cmd/bootz
+   cmd/cat
    cmd/cbsysinfo
    cmd/cls
    cmd/conitrace