HAL Timebase, Power, Clock: semaphore guarded access to clock and power modes, better sleep mode. (#307)

firmware/targets/f4/api-hal/api-hal-clock.h

@@ -0,0 +1,7 @@
+#pragma once
+
+/* Switch to HSI clock */
+void api_hal_clock_switch_to_hsi();
+
+/* Switch to PLL clock */
+void api_hal_clock_switch_to_pll();