update overclock options

Due to some people reporting SD card corruption, back off on overclocking
core_freq for all overclock options other than turbo. For turbo, bump SDRAM
overclock to 600 and warn the user that some people have reported SD card
corruption.

raspi-config

@@ -211,32 +211,36 @@ do_overclock() {
   whiptail --msgbox "\
 Be aware that overclocking may reduce the lifetime of your
 Raspberry Pi. If overclocking at a certain level causes
-system instability, try a more modest overclock. See
-http://elinux.org/RPi_Overclocking for more information.\
+system instability, try a more modest overclock. Hold down
+shift during boot to temporarily disable overclock.
+See http://elinux.org/RPi_Overclocking for more information.\
 " 20 70 1
   OVERCLOCK=$(whiptail --menu "Chose overclock preset" 20 60 10 \
     "None" "700MHz ARM, 250MHz core, 400MHz SDRAM, 0 overvolt" \
-    "Modest" "800MHz ARM, 300MHz core, 400MHz SDRAM, 0 overvolt" \
-    "Medium" "900MHz ARM, 333MHz core, 450MHz SDRAM, 2 overvolt" \
-    "High" "950MHz ARM, 450MHz core, 450MHz SDRAM, 6 overvolt" \
-    "Turbo" "1000MHz ARM, 500MHz core, 500MHz SDRAM, 6 overvolt" \
+    "Modest" "800MHz ARM, 250MHz core, 400MHz SDRAM, 0 overvolt" \
+    "Medium" "900MHz ARM, 250MHz core, 450MHz SDRAM, 2 overvolt" \
+    "High" "950MHz ARM, 250MHz core, 450MHz SDRAM, 6 overvolt" \
+    "Turbo" "1000MHz ARM, 500MHz core, 600MHz SDRAM, 6 overvolt" \
     3>&1 1>&2 2>&3)
   if [ $? -eq 0 ]; then
     case "$OVERCLOCK" in
       None)
         set_overclock None 700 250 400 0
         ;;
       Modest)
-        set_overclock Modest 800 300 400 0
+        set_overclock Modest 800 250 400 0
         ;;
       Medium)
-        set_overclock Medium 900 333 450 2
+        set_overclock Medium 900 250 450 2
         ;;
       High)
-        set_overclock High 950 450 450 6
+        set_overclock High 950 250 450 6
         ;;
       Turbo)
-        set_overclock Turbo 1000 500 500 6
+        whiptail --msgbox "Warning: some people have reported SD card corruption with this level of overclock." \
+          20 70 1
+        [ $? -ne 0 ] && return 1
+        set_overclock Turbo 1000 500 600 6
         ;;
       *)
         whiptail --msgbox "Programmer error, unrecognised overclock preset" 20 60 2