boot/arm-trusted-firmware: don't enable SSP by default

SSP support requires support in ATF platform code. Not all platforms
implement plat_get_stack_protector_canary() hook. The result is build
failure:

(.text.asm.update_stack_protector_canary+0x4): undefined reference to `plat_get_stack_protector_canary'

Commit cf176128ec4 ("boot/arm-trusted-firmware: add SSP option")
originally introduces this issue. But then commit ccac9a5bbbd
("boot/arm-trusted-firmware: don't force ENABLE_STACK_PROTECTOR") hid
the problem by effectively disabling SSP for all platforms. So only
after commit 09acc7cbc91f5 ("boot/arm-trusted-firmware: fix SSP
support") the issue showed up.

Make SSP an opt-in for platform that actually provide the
plat_get_stack_protector_canary() hook.

Cc: Sergey Matyukevich <geomatsi@gmail.com>
Cc: Dick Olsson <hi@senzilla.io>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/arm-trusted-firmware/Config.in

@@ -212,7 +212,6 @@ config BR2_TARGET_ARM_TRUSTED_FIRMWARE_NEEDS_ARM32_TOOLCHAIN
 
 config BR2_TARGET_ARM_TRUSTED_FIRMWARE_SSP
 	bool "Build with SSP"
-	default y
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	depends on !BR2_SSP_NONE
 	help