Kobo: Yet another attempt at working around the hangs on the latest NXP boards (#10771)

* Notification: Drop the fencing from #10083; it never actually helped, and had subtle side-effects we could do without.
* VirtualKeyBoard: Flash on close, otherwise, some of the fast refresh glitches may be burned into the working buffer until a flash. Making sure we flash ourselves prevent it from sticking around on the page ;).
* util: Move `writeToSysfs` to base (i.e., `ffi/util`), as we need it there (and it actually makes more sense there anyway ;p).
* Bump base for https://github.com/koreader/koreader-base/pull/1645, which is where the actual workaround (hopefully) lives.

Re #8414, #9806, #10558

frontend/ui/widget/inputtext.lua

@@ -683,9 +683,12 @@ function InputText:onShowKeyboard(ignore_first_hold_release)
 end
 
 function InputText:onCloseKeyboard()
-    UIManager:close(self.keyboard)
     Device:stopTextInput()
-    self.is_keyboard_hidden = true
+
+    if not self.is_keyboard_hidden then
+        UIManager:close(self.keyboard)
+        self.is_keyboard_hidden = true
+    end
 end
 
 function InputText:onCloseWidget()