vlib: add a "vpplog" debug CLI

To add arbitrary text to the vlib log. Combines nicely with
comment/uncomment and the macro expander:

define MY_FEATURE uncomment   # or comment
...
$(MY_FEATURE) { vpplog { My feature was enabled } }

Type: improvement

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ia019f0a8fa670d8593ae01595f5ef410796e5b1c

src/vlib/cli.h

@@ -41,6 +41,7 @@
 #define included_vlib_cli_h
 
 #include <vppinfra/format.h>
+#include <vlib/log.h>
 
 struct vlib_cli_command_t;
 
@@ -152,6 +153,10 @@ typedef struct vlib_cli_main_t
     (struct vlib_cli_main_t *, u32 id, int before_or_after);
   void (**perf_counter_cbs_tmp)
     (struct vlib_cli_main_t *, u32 id, int before_or_after);
+
+  /* cli log */
+  vlib_log_class_t log;
+
 } vlib_cli_main_t;
 
 #ifndef CLIB_MARCH_VARIANT