boot/at91dataflashboot: disable stack-protector

Disable stack-protector to avoid the following build failure:

/home/buildroot/autobuild/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabi-ld -T elf32-littlearm.lds -Ttext 0 -n -o DataflashBoot-1.05.out objs/cstartup_ram.o objs/at45.o objs/com.o objs/dataflash.o objs/div0.o objs/init.o objs/main.o objs/stdio.o objs/asm_isr.o objs/jump.o objs/_udivsi3.o objs/_umodsi3.o objs/led.o
/home/buildroot/autobuild/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabi-ld: objs/main.o: in function `main':
main.c:(.text.startup+0x834): undefined reference to `__stack_chk_guard'

Fixes:
 - http://autobuild.buildroot.org/results/502c54be4978e628724d72ee2a75d4c5e0a6ace8

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit d868d09d3a0c0db2c5112964139e5ecae8b1da51)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/at91dataflashboot/at91dataflashboot.mk

@@ -12,7 +12,8 @@ AT91DATAFLASHBOOT_INSTALL_TARGET = NO
 AT91DATAFLASHBOOT_INSTALL_IMAGES = YES
 
 define AT91DATAFLASHBOOT_BUILD_CMDS
-	make -C $(@D) CROSS_COMPILE=$(TARGET_CROSS)
+	make -C $(@D) CROSS_COMPILE=$(TARGET_CROSS) \
+		CFLAGS="$(TARGET_CFLAGS) -fno-stack-protector"
 endef
 
 define AT91DATAFLASHBOOT_INSTALL_IMAGES_CMDS