Some more file/path restructuring.

All #includes now explicitly use the "<libopencm3/stm32/rcc.h>" format.
If you want to get rid of the "libopencm3" prefix in your local project you
can add a respective -I entry in your Makefile (not recommended though).

All .ld files and .a libs are installed in $(TOOLCHAIN_DIR)/lib
directly (as before), but are now renamed to avoid potential
conflicts now or in the future. Examples:

  libopencm3_lpc13xx.a
  libopencm3_lpc13xx.ld
  libopencm3_stm32.a
  libopencm3_stm32.ld

lib/usb/usb_control.c

@@ -18,7 +18,7 @@
  */
 
 #include <stdlib.h>
-#include <usb/usbd.h>
+#include <libopencm3/usb/usbd.h>
 #include "usb_private.h"
 
 static struct usb_control_state {