arm-trusted-firmware: allow to generate the BL31 image

Some platforms (e.g Allwinner ARM64) don't build a FIP image out of
ATF, but only a BL31 image, which is then encapsulated in U-Boot. This
commit adds an ATF option to build such an image.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/arm-trusted-firmware/Config.in

@@ -64,6 +64,13 @@ config BR2_TARGET_ARM_TRUSTED_FIRMWARE_FIP
 	  platforms were ATF encapsulates the second stage bootloader
 	  (such as U-Boot).
 
+config BR2_TARGET_ARM_TRUSTED_FIRMWARE_BL31
+	bool "Build BL31 image"
+	help
+	  This option enables building the BL31 image. This is
+	  typically used on platforms where another bootloader (e.g
+	  U-Boot) encapsulates ATF BL31.
+
 config BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_AS_BL33
 	bool "Use U-Boot as BL33"
 	depends on BR2_TARGET_UBOOT

boot/arm-trusted-firmware/arm-trusted-firmware.mk

@@ -47,6 +47,10 @@ ifeq ($(BR2_TARGET_ARM_TRUSTED_FIRMWARE_FIP),y)
 ARM_TRUSTED_FIRMWARE_MAKE_TARGETS += fip
 endif
 
+ifeq ($(BR2_TARGET_ARM_TRUSTED_FIRMWARE_BL31),y)
+ARM_TRUSTED_FIRMWARE_MAKE_TARGETS += bl31
+endif
+
 define ARM_TRUSTED_FIRMWARE_BUILD_CMDS
 	$(TARGET_CONFIGURE_OPTS) \
 		$(MAKE) -C $(@D) $(ARM_TRUSTED_FIRMWARE_MAKE_OPTS) \