flags: do not override CFLAGS from host

Some build systems set CFLAGS in environment before the build, but they
got overridden by CFLAGS immediate assignment operator.

Replace immediate assignment with append directive and use override
directive, so the former definition of CFLAGS gets passed either from
command line or via environment is completely expanded (see [1] for
detailed explanation of override directive and variable append).

Link: [1] https://www.gnu.org/software/make/manual/html_node/Override-Directive.html

Signed-off-by: Andrey Zhizhikin <[email protected]>
Reviewed-by: Jerome Forissier <[email protected]>
Reviewed-by: Joakim Bech <[email protected]>

Author: zandrey

flags.mk

@@ -6,7 +6,7 @@ CROSS_COMPILE   ?= arm-linux-gnueabihf-
 CC              ?= $(CROSS_COMPILE)gcc
 AR		?= $(CROSS_COMPILE)ar
 
-CFLAGS          := -Wall -Wbad-function-cast -Wcast-align \
+override CFLAGS += -Wall -Wbad-function-cast -Wcast-align \
 		   -Werror-implicit-function-declaration -Wextra \
 		   -Wfloat-equal -Wformat-nonliteral -Wformat-security \
 		   -Wformat=2 -Winit-self -Wmissing-declarations \
@@ -16,13 +16,13 @@ CFLAGS          := -Wall -Wbad-function-cast -Wcast-align \
 		   -Wswitch-default -Wunsafe-loop-optimizations \
 		   -Wwrite-strings -D_FILE_OFFSET_BITS=64
 ifeq ($(CFG_WERROR),y)
-CFLAGS		+= -Werror
+override CFLAGS	+= -Werror
 endif
-CFLAGS          += -c -fPIC
+override CFLAGS += -c -fPIC
 
 DEBUG       ?= 0
 ifeq ($(DEBUG), 1)
-CFLAGS          += -DDEBUG -O0 -g
+override CFLAGS += -DDEBUG -O0 -g
 endif
 
 RM              := rm -f