mk/aosp_optee.mk: define OPTEE_BIN for path of tee.bin

so that other android projects could use OPTEE_BIN
as the dependency instead of the old BUILD_OPTEE_OS target.

Ths is workaround for the problem with following with aosp master:
external/optee_test/Android.mk: error: xtest: LOCAL_ADDITIONAL_DEPENDENCIES must only contain paths (not module names)

Reviewed-by: Victor Chong <victor.chong@linaro.org>

Tested by: Victor Chong <victor.chong@linaro.org> (hikey aosp)
Tested-by: Yongqin Liu <yongqin.liu@linaro.org> (hikey aosp master)

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>

mk/aosp_optee.mk

@@ -34,11 +34,19 @@ CROSS_COMPILE32 := $(TOP_ROOT_ABS)/$($(combo_2nd_arch_prefix)TARGET_TOOLS_PREFIX
 CROSS_COMPILE_LINE += CROSS_COMPILE32="$(CROSS_COMPILE32)"
 endif
 
-##########################################################
-## define BUILD_OPTEE_OS target, add condition check    ##
-## to make it only be defined once even though          ##
-## this file might be included multiple times           ##
-##########################################################
+OPTEE_BIN := $(TOP_ROOT_ABS)/$(OPTEE_OS_OUT_DIR)/core/tee.bin
+
+$(OPTEE_BIN) : BUILD_OPTEE_OS
+
+###########################################################
+## define BUILD_OPTEE_OS target, add condition check     ##
+## to make it only be defined once even though           ##
+## this file might be included multiple times            ##
+## This BUILD_OPTEE_OS will help to generate the header  ##
+## files under $(TA_DEV_KIT_DIR)/host_include and        ##
+## the $(OPTEE_BIN) file which will be used as dependency##
+## for other projects                                    ##
+###########################################################
 ifneq (true,$(BUILD_OPTEE_OS_DEFINED))
 BUILD_OPTEE_OS_DEFINED := true