cmd: Update the memory-search command

Add various fixes and improvements to this command that were missed in
the original version. Unfortunately I forgot to send v2.

- Fix Kconfig name
- Use a separate variable for the remaining search length
- Correct a minor bug
- Move into a separate test suite
- Add -q flag to the 'quiet' test to test operation when console is enabled
- Enable the feature for sandbox

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