buildscript: add color to the root warning

Use color output when the `tput` command is available.
`tput` handles terminal-dependant capabilities, so the script should
remain portable.

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>

buildscript

@@ -381,8 +381,12 @@ buildall() {
 }
 
 if [ "$(/usr/bin/id -u)" -eq 0 ]; then
-    echo "don't run buildscript as root"
-    exit 1
+    TPUT=$(command -v tput || echo ":")
+    textWarn="$($TPUT bold setaf 1)" # bold + red
+    textReset="$($TPUT sgr0)"        # be a good citizen
+
+    printf "${textWarn}WARNING: buildscript is running as root${textReset}\n"
+    printf "${textWarn}WARNING: build may fail at a later stage${textReset}\n"
 fi
 
 if [ "$1" != "selectbsp" -a "$1" != "selectvariant" ]; then