bootcache: remove Wformat from common.mk

The Makefile has "CFLAGS += -Wformat=0", the common.mk has
"COMMON_CFLAGS :=  -Wall -Werror -fno-strict-aliasing -O1  -Wformat=2"
This one overides the CFLAGS in make file and cause clang syntax check
fail.
BUG=chromium:230617
TEST=CFLAGS="-clang -print-cmdline" CXXFLAGS="-clang -print-cmdline"
      emerge-x86-alex bootcache passes

Change-Id: Ia68b70f890dad8152b741d2ee39724a1fe412fc8
Reviewed-on: https://gerrit.chromium.org/gerrit/47915
Reviewed-by: Paul Taysom <taysom@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
diff --git a/common.mk b/common.mk
index 058e906..7533291 100644
--- a/common.mk
+++ b/common.mk
@@ -279,7 +279,7 @@
 COMMON_CFLAGS-gcc := -fstack-protector-strong -fvisibility=internal -ggdb3 \
   -Wa,--noexecstack
 COMMON_CFLAGS-clang := -fstack-protector-all -fvisibility=hidden -ggdb
-COMMON_CFLAGS := -Wall -Werror -fno-strict-aliasing -O1 -Wformat=2
+COMMON_CFLAGS := -Wall -Werror -fno-strict-aliasing -O1
 CXXFLAGS += $(COMMON_CFLAGS) $(COMMON_CFLAGS-$(CXXDRIVER))
 CFLAGS += $(COMMON_CFLAGS) $(COMMON_CFLAGS-$(CDRIVER))
 CPPFLAGS += -D_FORTIFY_SOURCE=2