Use slightly flatter directory structure since some developers did not

like too many folders.
Extracted code from some files into separate files to fit with the new
layout.

src/drivers/system_common.h

@@ -0,0 +1,17 @@
+#pragma once
+
+void systemInit(void);
+void delayMicroseconds(uint32_t us);
+void delay(uint32_t ms);
+
+uint32_t micros(void);
+uint32_t millis(void);
+
+// failure
+void failureMode(uint8_t mode);
+
+// bootloader/IAP
+void systemReset(bool toBootloader);
+
+// current crystal frequency - 8 or 12MHz
+extern uint32_t hse_value;