binman: Indicate how to make binman verbose

Add notes about how to make binman produce verbose logging when building.

Add a comment on how to do this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 6cdd367..655de41 100644
--- a/Makefile
+++ b/Makefile
@@ -1330,6 +1330,7 @@
 # binman
 # ---------------------------------------------------------------------------
 # Use 'make BINMAN_DEBUG=1' to enable debugging
+# Use 'make BINMAN_VERBOSE=3' to set vebosity level
 default_dt := $(if $(DEVICE_TREE),$(DEVICE_TREE),$(CONFIG_DEFAULT_DEVICE_TREE))
 quiet_cmd_binman = BINMAN  $@
 cmd_binman = $(srctree)/tools/binman/binman $(if $(BINMAN_DEBUG),-D) \