touch devices: Disable touch on softlock.

Target that have a touchpad/touchscreen should disable it while
being locked (In order to avoid LCD to drain battery power due to
"key locked" constant reporting messages. If they a have a keylock
button this was already handled at driver level. If not (e.g. fuze+),
they will have to implement a switch at driver level that action.c
can operate on softlock.
This patch does the following for any target having a touchpad
or a touchscreen and no HAS_BUTTON_HOLD (ie any softlock target)
1) it implements the code to call button_enable_touch(bool en) in
action.c.
2) button_enable_touch is implemented in button.c and call
either touchpad_enable or touchscreen_enable
3) those two function are implemented respectively in touchscreen.c
and a new touchpad.c file. They provide a generic way to silents touch's
device and call a function at driver level where target specific code
can be implemented if possible/needed (for power saving for instance).
Those function name are touchpad_enable_device and touchscreen_enable_device
4) we implement an empty function at driver level of targets that need it
to have them still being able to compiled.

Change-Id: I9ead78a25bd33466a8533f5b9f259b395cb5ce49
Reviewed-on: http://gerrit.rockbox.org/569
Reviewed-by: Thomas Martitz <kugel@rockbox.org>
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>

firmware/target/hosted/android/button-android.c

@@ -129,6 +129,11 @@ void button_init_device(void)
     set_rockbox_ready();
 }
 
+void touchscreen_enable_device(bool en)
+{
+    (void)en; /* FIXME: do something smart */
+}
+
 int button_read_device(int *data)
 {
     int btn = last_btns;