Tools: Checksum for gcc-arm download on arch prereqs

This will check to see if the tar.bz2 file exists and if it does it will
run a checksum and skip redownloading the file if its already there. If
the checksum fails or the file doesn't exist it will redownload the
file.

I ran into issues with the download taking so long that my sudo
permissions timed out and the install failed to complete. When rerunning
the script it would redownload the file even if the file was already
there. This change solves this issue.

Tools/environment_install/install-prereqs-arch.sh

@@ -33,6 +33,7 @@ PYTHON_PKGS="future lxml pymavlink MAVProxy pexpect argparse matplotlib pyparsin
 ARM_ROOT="gcc-arm-none-eabi-10-2020-q4-major"
 ARM_TARBALL="$ARM_ROOT-x86_64-linux.tar.bz2"
 ARM_TARBALL_URL="https://firmware.ardupilot.org/Tools/STM32-tools/$ARM_TARBALL"
+ARM_TARBALL_CHECKSUM="21134caa478bbf5352e239fbc6e2da3038f8d2207e089efc96c3b55f1edcd618" 
 
 # Ardupilot Tools
 ARDUPILOT_TOOLS="ardupilot/Tools/autotest"
@@ -85,9 +86,33 @@ pip3 -q install -U $PYTHON_PKGS
 if [ ! -d $OPT/$ARM_ROOT ]; then
     (
         cd $OPT;
-        sudo wget --progress=dot:giga $ARM_TARBALL_URL;
-        sudo tar xjf ${ARM_TARBALL};
-        sudo rm ${ARM_TARBALL};
+
+        # Check if file exists and verify checksum
+        download_required=false
+        if [ -e "$ARM_TARBALL" ]; then
+            echo "File exists. Verifying checksum..."
+
+            # Calculate the checksum of the existing file
+            ACTUAL_CHECKSUM=$(sha256sum "$ARM_TARBALL" | awk '{ print $1 }')
+
+            # Compare the actual checksum with the expected one
+            if [ "$ACTUAL_CHECKSUM" == "$ARM_TARBALL_CHECKSUM" ]; then
+                echo "Checksum valid. No need to redownload."
+            else
+                echo "Checksum invalid. Redownloading the file..."
+                download_required=true
+                sudo rm $ARM_TARBALL
+            fi
+        else
+            echo "File does not exist. Downloading..."
+            download_required=true
+        fi
+
+        if $download_required; then
+            sudo wget -O "$ARM_TARBALL" --progress=dot:giga $ARM_TARBALL_URL
+        fi
+
+        sudo tar xjf ${ARM_TARBALL}
     )
 fi