cmd: Add a memory-search command

It is useful to be able to find hex values and strings in a memory range.
Add a command to support this.

cmd: Fix 'md' and add a memory-search command
At present 'md.q' is broken. This series provides a fix for this. It also
implements a new memory-search command called 'ms'. It allows searching
memory for hex and string data.
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Signed-off-by: Simon Glass <sjg@chromium.org>