cmd: Move meminfo command into its own file

In preparation for expanding this command, move it into a separate file.
Rename the function to remove the extra underscore. Update the number of
arguments to 1, since 3 is incorrect.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 3ee70f3..ec2fe7a 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -885,6 +885,7 @@
 
 config CMD_MEMINFO
 	bool "meminfo"
+	default y if SANDBOX
 	help
 	  Display memory information.