boot/afboot-stm32: disable stack-protector

boot/afboot-stm32: disable stack-protector

Disable stack-protector to avoid the following build failure:

/home/autobuild/autobuild/instance-14/output-1/host/bin/arm-buildroot-linux-gnueabi-ld -T stm32f429.lds --gc-sections -o stm32f469i-disco.elf stm32f469i-disco.o gpio.o mpu.o qspi.o start_kernel.o usart-f4.o
/home/autobuild/autobuild/instance-14/output-1/host/bin/arm-buildroot-linux-gnueabi-ld: stm32f469i-disco.o: in function `main':
stm32f469i-disco.c:(.text.startup.main+0x3b0): undefined reference to `__stack_chk_guard'
/home/autobuild/autobuild/instance-14/output-1/host/bin/arm-buildroot-linux-gnueabi-ld: stm32f469i-disco.c:(.text.startup.main+0x4c0): undefined reference to `__stack_chk_fail'
/home/autobuild/autobuild/instance-14/output-1/host/bin/arm-buildroot-linux-gnueabi-ld: stm32f469i-disco.c:(.text.startup.main+0x4ec): undefined reference to `__stack_chk_guard'

Fixes:
 - http://autobuild.buildroot.org/results/fa6ffab24c3998f21034ab24e8d2852cacde08c1

Signed-off-by: Fabrice Fontaine <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>