Move command line API into cli.c

We now have a single entry point to the CLI, whether simple or hush. Put
this in its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index ae79865..81721f9 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -18,6 +18,7 @@
 # We always have this since drivers/ddr/fs/interactive.c needs it
 obj-y += cli_simple.o
 
+obj-y += cli.o
 obj-y += cli_readline.o
 obj-y += s_record.o
 obj-y += xyzModem.o