Boot time labs: simplify instructions to fetch sources

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

labs/boot-time-sources-download/boot-time-sources-download.tex

@@ -33,17 +33,6 @@ \section{Cloning the Buildroot source tree}
 git clone https://git.buildroot.net/buildroot
 \end{verbatim}
 
-If the connection to the Internet turns out to be not fast enough,
-your instructor can give you a USB flash drive with a
-\code{tar} archive of a recently cloned tree:
-
-\begin{verbatim}
-tar xf buildroot-git.tar.xz
-cd buildroot
-git checkout master
-git pull
-\end{verbatim}
-
 We will select a particular release tag later. Let's move on to the next
 source repository.
 
@@ -62,17 +51,10 @@ \section{Cloning the U-Boot source tree}
 git clone https://git.denx.de/u-boot.git
 \end{verbatim}
 
-Similarly, your instructor can give you a pre-downloaded archive if
-needed.
-
 \section{Cloning the mainline Linux tree}
 
 Go to the \code{$HOME/boot-time-labs/kernel} directory.
 
-This represents the biggest amount of sources to download, actually more
-than 1 GB of data! Again, you can use a pre-downloaded archive if that
-turns out to be to much for your actual connection.
-
 \begin{verbatim}
 git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
 \end{verbatim}
@@ -105,15 +87,6 @@ \section{Accessing stable Linux releases}
 \end{verbatim}
 }
 
-As this still represents many git objects to download (about 300 MiB when 5.4 was
-the latest version), if you are using an already downloaded git tree,
-your instructor will probably have fetched the {\em stable} branch ahead
-of time for you too. You can check by running:
-
-\begin{verbatim}
-git branch -a
-\end{verbatim}
-
 We will choose a particular stable version in the next labs.
 
 Now, let's continue the lectures. This will leave time for the commands