build system: Remove FLAGS_XXX variable option, replace with per-target overrides

Use for targeted disabling of warnings in LWIP.

components/lwip/component.mk

@@ -6,5 +6,7 @@ COMPONENT_ADD_INCLUDEDIRS := include/lwip include/lwip/port include/lwip/posix
 
 COMPONENT_SRCDIRS := api apps/sntp apps core/ipv4 core/ipv6 core netif port/freertos port/netif port
 
-CFLAGS += -Wno-address -Wno-unused-variable -Wno-unused-but-set-variable
+CFLAGS += -Wno-address  # lots of LWIP source files evaluate macros that check address of stack variables
 
+api/tcpip.o apps/dhcpserver.o: CFLAGS += -Wno-unused-variable
+apps/dhcpserver.o core/pbuf.o core/tcp_in.o: CFLAGS += -Wno-unused-but-set-variable

docs/build_system.rst

@@ -244,7 +244,12 @@ The following variables can be set inside ``component.mk`` to control the build
   settings. Component-specific additions can be made via ``CXXFLAGS
   +=``. It is also possible (although not recommended) to override
   this variable completely for a component.
-- ``FLAGS_basename`` allows you to set compilation flags to apply to a single source file only. For example, this can useful for disabling warnings in a single upstream source file. The ``basename`` portion is the directory (relative to ``COMPONENT_PATH``) and the base filename (without extension) of the source file. For example, if a file inside ``COMPONENT_PATH`` is ``library/alpha/widget.c`` then you can set variable ``FLAGS_library/alpha/widget := -DTEST`` to pass the TEST macro when compiling this source file only.
+
+To apply compilation flags to a single source file, you can add a variable override as a target, ie::
+
+  apps/dhcpserver.o: CFLAGS += -Wno-unused-variable
+
+This can be useful if there is upstream code that emits warnings.
 
 Component Configuration
 -----------------------

make/component_wrapper.mk

@@ -157,15 +157,15 @@ define GenerateCompileTargets
 # $(1) - directory containing source files, relative to $(COMPONENT_PATH)
 $(1)/%.o: $$(COMPONENT_PATH)/$(1)/%.c $(COMMON_MAKEFILES) $(COMPONENT_MAKEFILE) | $(1)
 	$$(summary) CC $$@
-	$$(CC) $$(CFLAGS) $$(CPPFLAGS) $$(FLAGS_$$(subst .o,,$$(@))) $$(addprefix -I ,$$(COMPONENT_INCLUDES)) $$(addprefix -I ,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
+	$$(CC) $$(CFLAGS) $$(CPPFLAGS) $$(addprefix -I ,$$(COMPONENT_INCLUDES)) $$(addprefix -I ,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
 
 $(1)/%.o: $$(COMPONENT_PATH)/$(1)/%.cpp $(COMMON_MAKEFILES) $(COMPONENT_MAKEFILE) | $(1)
 	$$(summary) CXX $$@
-	$$(CXX) $$(CXXFLAGS) $$(CPPFLAGS) $$(FLAGS_$$(subst .o,,$$(@))) $$(addprefix -I,$$(COMPONENT_INCLUDES)) $$(addprefix -I,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
+	$$(CXX) $$(CXXFLAGS) $$(CPPFLAGS) $$(addprefix -I,$$(COMPONENT_INCLUDES)) $$(addprefix -I,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
 
 $(1)/%.o: $$(COMPONENT_PATH)/$(1)/%.S $(COMMON_MAKEFILES) $(COMPONENT_MAKEFILE) | $(1)
 	$$(summary) AS $$@
-	$$(CC) $$(CPPFLAGS) $$(FLAGS_$$(subst .o,,$$(@))) $$(addprefix -I ,$$(COMPONENT_INCLUDES)) $$(addprefix -I ,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
+	$$(CC) $$(CPPFLAGS) $$(addprefix -I ,$$(COMPONENT_INCLUDES)) $$(addprefix -I ,$$(COMPONENT_EXTRA_INCLUDES)) -I$(1) -c $$< -o $$@
 
 # CWD is build dir, create the build subdirectory if it doesn't exist
 $(1):