boot/arm-trusted-firmware: add optional host-arm-gnu-a-toolchain dependency

boot/arm-trusted-firmware: add optional host-arm-gnu-a-toolchain dependency

Some ATF configurations, require a pre-built bare metal toolchain to
build some platforms which host cortex-m series core, for instance
rockchip rk3399 has a cortex-m0 core. Without a pre-built bare metal
toolchain, the build fails:

make[3]: arm-none-eabi-gcc: Command not found

To solve this, this commit implements a
BR2_TARGET_ARM_TRUSTED_FIRMWARE_NEEDS_ARM32_TOOLCHAIN
option. Platforms which have such requirement should enable this
config option.

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>