common: target optee-os-devkit builds the devkit

Add build target optee-os-devkit to get the TA devkit be built
and installed. It can be used as a dependency when building external
TAs that need to be linked to OP-TEE as early TAs in which case
building OP-TEE core depends on such TAs being already built.

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
diff --git a/common.mk b/common.mk
index 6716087..e3b0bb3 100644
--- a/common.mk
+++ b/common.mk
@@ -553,6 +553,10 @@
 optee-os-clean-common:
 	$(MAKE) -C $(OPTEE_OS_PATH) $(OPTEE_OS_COMMON_FLAGS) clean
 
+.PHONY: optee-os-devkit
+optee-os-devkit:
+	$(MAKE) -C $(OPTEE_OS_PATH) $(OPTEE_OS_COMMON_FLAGS) ta_dev_kit
+
 ################################################################################
 # fTPM Rules
 ################################################################################
@@ -570,7 +574,7 @@
 .PHONY: ftpm
 ftpm:
 ifeq ($(MEASURED_BOOT_FTPM),y)
-ftpm: optee-os
+ftpm: optee-os-devkit
 	$(FTPM_FLAGS) $(MAKE) -C $(FTPM_PATH)
 endif