Use generic OPTEE_OS_PLATFORM to define platform

Define OPTEE_OS_PLATFORM for each supported platform to define the
target platform for which OP-TEE OS package is built. It is used in
common.mk (and ti/ti-common.mk).

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/hikey.mk b/hikey.mk
index d20765f..4919642 100644
--- a/hikey.mk
+++ b/hikey.mk
@@ -15,6 +15,8 @@
 # eMMC flash size: 8 or 4 GB [default 8]
 CFG_FLASH_SIZE ?= 8
 
+OPTEE_OS_PLATFORM = hikey
+
 ################################################################################
 # Includes
 ################################################################################
@@ -191,9 +193,7 @@
 ################################################################################
 # OP-TEE
 ################################################################################
-OPTEE_OS_COMMON_FLAGS += PLATFORM=hikey \
-			CFG_CONSOLE_UART=$(CFG_SW_CONSOLE_UART)
-OPTEE_OS_CLEAN_COMMON_FLAGS += PLATFORM=hikey
+OPTEE_OS_COMMON_FLAGS += CFG_CONSOLE_UART=$(CFG_SW_CONSOLE_UART)
 
 .PHONY: optee-os
 optee-os: optee-os-common