xtest: add helper to convert UUID string to TEEC_UUID

This is copied and adapted from optee-os repository:

 lib/libutee/tee_uuid_from_str.c

Renamed as xtest_uuid_helpers.c|h

Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Vesa Jääskeläinen <vesa.jaaskelainen@vaisala.com>
diff --git a/host/xtest/Makefile b/host/xtest/Makefile
index 9f0998c..7ac0a3b 100644
--- a/host/xtest/Makefile
+++ b/host/xtest/Makefile
@@ -77,7 +77,8 @@
 	stats.c \
 	xtest_helpers.c \
 	xtest_main.c \
-	xtest_test.c
+	xtest_test.c \
+	xtest_uuid_helpers.c
 
 ifeq ($(CFG_SECSTOR_TA_MGMT_PTA),y)
 srcs += install_ta.c