binman: Allow selection of logging verbosity

Support a new BINMAN_VERBOSE option to the build, to allow passing the
-v flag to binman.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/binman/README b/tools/binman/README
index b4f6392..8e0f0a8 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -934,6 +934,12 @@
    make sandbox_defconfig
    make BINMAN_DEBUG=1
 
+To enable verbose logging from binman, base BINMAN_VERBOSE to your build, which
+adds a -v<level> option to the call to binman:
+
+   make sandbox_defconfig
+   make BINMAN_VERBOSE=5
+
 
 History / Credits
 -----------------