kbuild: drop V=0 support

The top Makefile sets KBUILD_VERBOSE to 0 by default, it looks weird
now because V=1 and V=2 can be OR'ed as V=12. The default should be
empty.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Documentation/kbuild/makefiles.rst

@@ -523,7 +523,7 @@ more details, with real examples.
 			$(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
 			@$(kecho) '  Kernel: $(boot)/$@ is ready'
 
-	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
+	When kbuild is executing with KBUILD_VERBOSE unset, then only a shorthand
 	of a command is normally displayed.
 	To enable this behaviour for custom commands kbuild requires
 	two variables to be set::
@@ -544,7 +544,7 @@ more details, with real examples.
 
 		  GEN     lib/crc32table.h
 
-	will be displayed with "make KBUILD_VERBOSE=0".
+	will be displayed with "make KBUILD_VERBOSE=".
 
 3.12 Command change detection
 -----------------------------

Makefile

@@ -78,9 +78,6 @@ unexport GREP_OPTIONS
 ifeq ("$(origin V)", "command line")
   KBUILD_VERBOSE = $(V)
 endif
-ifndef KBUILD_VERBOSE
-  KBUILD_VERBOSE = 0
-endif
 
 quiet = quiet_
 Q = @
@@ -102,7 +99,7 @@ endif
 
 ifneq ($(findstring s,$(short-opts)),)
 quiet=silent_
-KBUILD_VERBOSE = 0
+override KBUILD_VERBOSE :=
 endif
 
 export quiet Q KBUILD_VERBOSE
@@ -1774,7 +1771,7 @@ help:
 		printf "  %-16s - Show all of the above\\n" help-boards; \
 		echo '')
 
-	@echo  '  make V=n   [targets] 0: quiet build (default), 1: verbose build'
+	@echo  '  make V=n   [targets] 1: verbose build'
 	@echo  '                       2: give reason for rebuild of target'
 	@echo  '                       V=1 and V=2 can be combined with V=12'
 	@echo  '  make O=dir [targets] Locate all output files in "dir", including .config'