be more intelligent about inserting config options

If a config option is already in the file but commented out, modify that line
rather than inserting a new one.

debian/control

@@ -10,5 +10,5 @@ Homepage: https://github.com/asb/raspi-config
 
 Package: raspi-config
 Architecture: all
-Depends: ${misc:Depends}, whiptail, parted
+Depends: ${misc:Depends}, whiptail, parted, lua5.1
 Description: Simple configuration for Raspberry Pi

raspi-config

@@ -70,6 +70,28 @@ EOF
 The filesystem will be enlarged upon the next reboot" 20 60 2
 }
 
+set_config_var() {
+  lua - "$1" "$2" "$3" <<EOF > "$3.bak"
+local key=assert(arg[1])
+local value=assert(arg[2])
+local fn=assert(arg[3])
+local file=assert(io.open(fn))
+local made_change=false
+for line in file:lines() do
+  if line:match("^#?%s*"..key.."=.*$") then
+    line=key.."="..value
+    made_change=true
+  end
+  print(line)
+end
+
+if not made_change then
+  print(key.."="..value)
+end
+EOF
+mv "$3.bak" "$3"
+}
+
 # $1 is 0 to disable overscan, 1 to disable it
 set_overscan() {
   # Stop if /boot is not a mountpoint
@@ -81,13 +103,9 @@ set_overscan() {
 
   if [ "$1" -eq 0 ]; then # disable overscan
     sed /boot/config.txt -i -e "s/^overscan_/#overscan_/"
-    if grep -q "^disable_overscan" /boot/config.txt; then
+    set_config_var disable_overscan 1 /boot/config.txt
-      sed -i /boot/config.txt -e "s/^disable_overscan.*$/disable_overscan=1/"
-    else
-      printf "disable_overscan=1\n" >> /boot/config.txt
-    fi
   else # enable overscan
-    sed -i /boot/config.txt -e "s/^disable_overscan.*$/disable_overscan=0/"
+    set_config_var disable_overscan 0 /boot/config.txt
   fi
 }