buildsys: document UCLIBC_EXTRA_CPPFLAGS

Rename EXTRA_CPPFLAGS to UCLIBC_EXTRA_CPPFLAGS and make them override
previous flags (by appending - not prepending - them to the other flags).

Makefile.help

@@ -60,4 +60,5 @@ help:
 	@echo '			  and the include dir (usually "/usr")'
 	@echo '  MULTILIB_DIR=		- Directory component for libraries (default "lib").'
 	@echo '  UCLIBC_EXTRA_CFLAGS	- extra CFLAGS for compiling uClibc'
+	@echo '  UCLIBC_EXTRA_CPPFLAGS	- extra CPPFLAGS for compiling uClibc'
 

Makerules

@@ -233,12 +233,12 @@ cmd_compile.c = $(CC) -c $< -o $@ $(CFLAGS) \
 	$(CFLAGS-$(notdir $<)) \
 	$(CFLAGS-$(notdir $@)) \
 	$(CFLAGS_gen.dep)
-cmd_compile.i = $(cmd_compile.c:-c=-E -dD $(EXTRA_CPPFLAGS))
+cmd_compile.i = $(cmd_compile.c:-c=-E -dD) $(UCLIBC_EXTRA_CPPFLAGS)
 cmd_compile.s = $(cmd_compile.c:-c=-S)
 cmd_compile.u = $(CC) $^ $(DEPS-$(notdir $@)) -o $@ $(CFLAGS) $(CFLAGS-$(notdir $(^D))) $(CFLAGS-$(notdir $@)) $(CFLAGS_gen.dep)
 cmd_compile.S = $(filter-out -std=gnu99, $(cmd_compile.c)) -D__ASSEMBLER__ $(ASFLAGS) $(ARCH_ASFLAGS) $(ASFLAGS-$(suffix $@)) $(ASFLAGS-$(notdir $<)) $(ASFLAGS-$(notdir $@))
 cmd_compile.m = $(cmd_compile.c) -DL_$(patsubst %$(suffix $(notdir $@)),%,$(notdir $@))
-cmd_compile.mi= $(cmd_compile.m:-c=-E -dD $(EXTRA_CPPFLAGS))
+cmd_compile.mi= $(cmd_compile.m:-c=-E -dD) $(UCLIBC_EXTRA_CPPFLAGS)
 
 cmd_compile-m = $(CC) $^ -c -o $@ $(CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(notdir $(@D))) $(CFLAGS-$(notdir $@)) $(sort $(foreach d,$(^:$(top_srcdir)=),$(collect_multi_flags)))
 cmd_strip     = $(STRIPTOOL) $(STRIP_FLAGS) $^