boot/arm-trusted-firmware: Forward stack protection configuration

boot/arm-trusted-firmware: Forward stack protection configuration

TF-A supports stack smashing protection (-fstack-protector-*).
However, that feature is currently silently disabled because
ENABLE_STACK_PROTECTOR is not set during build time.

As documented in the TF-A user guide, the flag ENABLE_STACK_PROTECTOR
is required to enable stack protection support. When enabled the symbols
for the stack protector (e.g. __stack_chk_guard) are built.
This needs to be done because TF-A does not link against an external
library that provides that symbols (e.g. libc).

So in case we see that BR2_SSP_* is enabled, let's enable the corresponding
ENABLE_STACK_PROTECTOR build flag for TF-A as documented in the TF-A user guide.

This patch also fixes a the following linker errors with older TF-A versions
if BR2_SSP_* is enabled (i.e. -fstack-protector-* is used as compiler flag)
and ENABLE_STACK_PROTECTOR is not set, which are caused by the missing
stack protector symbols:

  [...]
  params_setup.c:(.text.params_early_setup+0xc): undefined reference to `__stack_chk_guard'
  aarch64-none-linux-gnu-ld: params_setup.c:(.text.params_early_setup+0x14): undefined reference to `__stack_chk_guard'
  aarch64-none-linux-gnu-ld: params_setup.c:(.text.params_early_setup+0x104): undefined reference to `__stack_chk_guard'
  aarch64-none-linux-gnu-ld: params_setup.c:(.text.params_early_setup+0x118): undefined reference to `__stack_chk_fail'
  aarch64-none-linux-gnu-ld: ./build/px30/release/bl31/pmu.o: in function `rockchip_soc_sys_pwr_dm_suspend':
  pmu.c:(.text.rockchip_soc_sys_pwr_dm_suspend+0xc): undefined reference to `__stack_chk_guard'
  [...]

TF-A releases after Nov 2019, that include 7af195e29a4, will circumvent
these issue by explicitliy and silently disabling the stack protector
by appending '-fno-stack-protector' to the compiler flags in case
ENABLE_STACK_PROTECTOR is not set.

Tested on a Rockchip PX30 based system (TF-A v2.2 and upstream/master).

Signed-off-by: Christoph Müllner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>