fix(sc7180): specify the workspace path exactly

Anchors the path to be an absolute path, making it resiliant to changes
in where make is invoked from.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I6fb2871f784171f77cdafba3eaeba3130800e874
diff --git a/tf_config/sc7180-default b/tf_config/sc7180-default
index 1ef99fe..fdc1d44 100644
--- a/tf_config/sc7180-default
+++ b/tf_config/sc7180-default
@@ -1,4 +1,4 @@
 COREBOOT=1
 CROSS_COMPILE=aarch64-none-elf-
 PLAT=sc7180
-QTISECLIB_PATH=../workspace/libqtisec.a
+QTISECLIB_PATH=${workspace}/libqtisec.a