ebtables: fix passing CFLAGS to configure

EBTABLES_K64U32 is a string passed directly to the configure script,
'+=' operator doesn't have any special meaningi inside it, so
CFLAGS+=-DKERNEL_64_USERSPACE_32 is passed to shell, overwriting previous
CFLAGS value.

Replace CFLAGS+="-DKERNEL_64_USERSPACE_32" with -DKERNEL_64_USERSPACE_32
and pass CFLAGS="$(TARGET_CFLAGS) $(EBTABLES_K64U32)".

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ebtables/ebtables.mk

@@ -10,11 +10,11 @@ EBTABLES_SITE = http://downloads.sourceforge.net/project/ebtables/ebtables/ebtab
 EBTABLES_LICENSE = GPLv2+
 EBTABLES_LICENSE_FILES = COPYING
 EBTABLES_STATIC = $(if $(BR2_PREFER_STATIC_LIB),static)
-EBTABLES_K64U32 = $(if $(BR2_KERNEL_64_USERLAND_32),CFLAGS+="-DKERNEL_64_USERSPACE_32")
+EBTABLES_K64U32 = $(if $(BR2_KERNEL_64_USERLAND_32),-DKERNEL_64_USERSPACE_32)
 
 define EBTABLES_BUILD_CMDS
 	$(MAKE) $(TARGET_CONFIGURE_OPTS) LIBDIR=/lib/ebtables $(EBTABLES_STATIC) \
-		$(EBTABLES_K64U32) -C $(@D)
+		CFLAGS="$(TARGET_CFLAGS) $(EBTABLES_K64U32)" -C $(@D)
 endef
 
 ifeq ($(BR2_PREFER_STATIC_LIB),y)