TF-M: Update Cmake to 3.15

The new build system for TF-M going to merge, it requires Cmake later
than 3.15.0.

Change-Id: I56efdb309cdfa4bac2d337047fd3f0b2cbfb439f
Signed-off-by: Karl Zhang <karl.zhang@arm.com>
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 1c3c5af..4dc380b 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -78,7 +78,7 @@
     curl --create-dirs -fsSLo /tmp/gcc-arm-none-eabi-linux.tar.bz2 'https://developer.arm.com/-/media/Files/downloads/gnu-rm/7-2018q2/gcc-arm-none-eabi-7-2018-q2-update-linux.tar.bz2?revision=bc2c96c0-14b5-4bb4-9f18-bceb4050fee7?product=GNU%20Arm%20Embedded%20Toolchain%20Downloads,64-bit,,Linux,7-2018-q2-update' ;\
     tar -xf /tmp/gcc-arm-none-eabi-linux.tar.bz2 -C /home/buildslave/tools --strip-components=1 ;\
     # Install CMake
-    curl --create-dirs -fsSLo /tmp/cmake-Linux-x86_64.tar.gz https://cmake.org/files/v3.14/cmake-3.14.0-Linux-x86_64.tar.gz ;\
+    curl --create-dirs -fsSLo /tmp/cmake-Linux-x86_64.tar.gz https://cmake.org/files/v3.15/cmake-3.15.0-Linux-x86_64.tar.gz ;\
     tar -xf /tmp/cmake-Linux-x86_64.tar.gz -C /home/buildslave/tools --strip-components=1 ;\
     # Fix permissions
     chmod 0755 /usr/share/jenkins ;\