Treat 'implicit fallthrough' errors as warnings

After fixing the warnings, the compiler option should be removed.

Issue #3307

repos/base-nova/src/kernel/nova/target.mk

@@ -16,6 +16,10 @@ CC_WARN          = -Wall -Wextra -Waggregate-return -Wcast-align -Wcast-qual \
                    -Wold-style-cast -Woverloaded-virtual -Wsign-promo \
                    -Wlogical-op -Wstrict-null-sentinel \
                    -Wstrict-overflow=5 -Wvolatile-register-var
+
+# XXX fix the warnings and remove this option
+CC_WARN         += -Wno-error=implicit-fallthrough
+
 CC_OPT          += -pipe \
                    -fdata-sections -fomit-frame-pointer -freg-struct-return \
                    -freorder-blocks -funit-at-a-time -fno-exceptions -fno-rtti \

repos/base/mk/global.mk

@@ -134,6 +134,11 @@ endif
 CC_OLEVEL ?= -O2
 CC_WARN   ?= -Wall
 
+#
+# XXX fix the warnings and remove this option
+#
+CC_WARN += -Wno-error=implicit-fallthrough
+
 #
 # Additional warnings for C++
 #

repos/ports/src/app/seoul/target.mk

@@ -26,6 +26,10 @@ include $(call select_from_repositories,lib/mk/libc-common.inc)
 
 CC_CXX_WARN_STRICT = -Wextra -Weffc++ -Werror
 CC_WARN += -Wno-parentheses -Wall -Wno-unused
+
+# XXX fix the warnings and remove this option
+CC_WARN += -Wno-error=implicit-fallthrough
+
 CC_CXX_OPT += -march=core2
 CC_OPT_model/intel82576vf := -mssse3
 CC_OPT_PIC :=