Minimal OP-TEE without user TAs

Hide all user TA related code under CFG_WITH_USER_TA. When compiled
with:
CFG_WITH_USER_TA=n
CFG_CRYPTO=n
CFG_ENC_FS=n
CFG_SE_API=n
CFG_PCSC_PASSTHRU_READER_DRV=n

Skips building in static TA tests for features not enabled.

The size of OP-TEE is reduced to one third of its original size.

Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (QEMU xtest 1001)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/Makefile b/Makefile
index fb4dcce..7911989 100644
--- a/Makefile
+++ b/Makefile
@@ -58,11 +58,13 @@
 # Platform config is supposed to assign the targets
 ta-targets ?= user_ta
 
+ifeq ($(CFG_WITH_USER_TA),y)
 define build-ta-target
 ta-target := $(1)
 include ta/ta.mk
 endef
 $(foreach t, $(ta-targets), $(eval $(call build-ta-target, $(t))))
+endif
 
 .PHONY: clean
 clean: