CI: Update to LLVM-13

- Switch sources and CI scripts to install and use LLVM-13
- Update to latest "focal" tag.

Signed-off-by: Tom Rini <trini@konsulko.com>

tools/docker/Dockerfile

@@ -2,7 +2,7 @@
 # This Dockerfile is used to build an image containing basic stuff to be used
 # to build U-Boot and run our test suites.
 
-FROM ubuntu:focal-20210827
+FROM ubuntu:focal-20210921
 MAINTAINER Tom Rini <trini@konsulko.com>
 LABEL Description=" This image is for building U-Boot inside a container"
 
@@ -12,7 +12,7 @@ ENV DEBIAN_FRONTEND=noninteractive
 # Add LLVM repository
 RUN apt-get update && apt-get install -y gnupg2 wget xz-utils && rm -rf /var/lib/apt/lists/*
 RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
-RUN echo deb http://apt.llvm.org/focal/ llvm-toolchain-focal-12 main | tee /etc/apt/sources.list.d/llvm.list
+RUN echo deb http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main | tee /etc/apt/sources.list.d/llvm.list
 
 # Manually install the kernel.org "Crosstool" based toolchains for gcc-11.1.0
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/11.1.0/x86_64-gcc-11.1.0-nolibc-aarch64-linux.tar.xz | tar -C /opt -xJ
@@ -39,7 +39,7 @@ RUN apt-get update && apt-get install -y \
 	binutils-dev \
 	bison \
 	build-essential \
-	clang-12 \
+	clang-13 \
 	coreutils \
 	cpio \
 	cppcheck \