Makefile: Support three levels of verbosity

At present the EC Makefile supports two levels of verbosity:

V unset: Show an abbreviated build log (operation and file only)
V=1: Show the full build log, including all commands

However, even the abbreviated build log includes a lot of output. It is
basically a long list of filenames and is of little use during
development. It is more useful to show just warnings and errors.

Add a new setting, V=0, which provides this.

BUG=chromium:680243
BRANCH=none
TEST=emerge-reef chromeos-ec; test each of V=, V=0, V=1 and see that the
Makefile does the correct thing.
Change-Id: I85c0423c5299fa3ab624ed9f7f7b6b7f73236611
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/427363
Reviewed-by: Randall Spangler <rspangler@chromium.org>

Makefile.rules

@@ -29,10 +29,24 @@ _dir_create := $(foreach d,$(dirs),$(shell [ -d $(out)/$(BLD)/$(d) ] || \
 _dir_y_create := $(foreach d,$(dirs-y),$(shell [ -d $(out)/$(BLD)/$(d) ] || \
 	    mkdir -p $(out)/RO/$(d); mkdir -p $(out)/RW/$(d)))
 
-# Decrease verbosity unless you pass V=1
-quiet = $(if $(V),,@echo '  $(2)' $(subst $(out)/,,$@) ; )$(cmd_$(1))
-silent = $(if $(V),,1>/dev/null)
-silent_err = $(if $(V),,2>/dev/null)
+# V unset for normal output, V=1 for verbose output, V=0 for silent build
+# (warnings/errors only). Use echo thus: $(call echo,"stuff  to  echo")
+ifeq ($(V),0)
+Q := @
+quiet = echo -n; $(cmd_$(1))
+silent = 1>/dev/null
+silent_err = 2>/dev/null
+else
+ifeq ($(V),)
+Q := @
+quiet = @echo '  $(2)' $(subst $(out)/,,$@) ; $(cmd_$(1))
+silent = 1>/dev/null
+silent_err = 2>/dev/null
+else
+Q :=
+quiet = $(cmd_$(1))
+endif
+endif
 
 # commands to build all targets
 cmd_lds = $(CPP) -P -C -MMD -MF $@.d -MT $@ $(CPPFLAGS) $< -o $@