Focus: Fix the focusHook

The `focusHook` function ought to be static, so instead of using `HostOS`
internals, use the accessors instead.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/Kaleidoscope/HostOS-Base.cpp

@@ -59,10 +59,10 @@ bool Base::focusHook(const char *command) {
     return false;
 
   if (Serial.peek() == '\n') {
-    Serial.println(os_);
+    Serial.println(::HostOS.os());
   } else {
     uint8_t new_os = Serial.parseInt();
-    os((Type) new_os);
+    ::HostOS.os((Type) new_os);
   }
 
   Serial.read();

src/Kaleidoscope/HostOS-Base.h

@@ -39,7 +39,7 @@ class Base : public KaleidoscopePlugin {
   Type os(void);
   void os(Type new_os);
 
-  bool focusHook(const char *command);
+  static bool focusHook(const char *command);
 
  protected:
   virtual void autoDetect(void) {}