stm32:rng: f4 rng peripheral is common with at least f2

Pull it up as common code immediately. Rename to v1, extract to common
with a doxygen marker stubs, add to F2 makefiles.

lib/stm32/f2/Makefile

@@ -44,6 +44,7 @@ OBJS            += crc_common_all.o dac_common_all.o dma_common_f24.o \
 		   timer_common_f24.o usart_common_all.o usart_common_f124.o \
 		   flash_common_f234.o flash_common_f24.o hash_common_f24.o \
 		   crypto_common_f24.o exti_common_all.o rcc_common_all.o
+OBJS		+= rng_common_v1.o
 
 OBJS            += usb.o usb_standard.o usb_control.o usb_fx07_common.o \
                    usb_f107.o usb_f207.o usb_msc.o

lib/stm32/f2/rng.c

@@ -0,0 +1,31 @@
+/* This file is used for documentation purposes. It does not need
+to be compiled. All source code is in the common area.
+If there is any device specific code required it can be included here,
+in which case this file must be added to the compile list. */
+
+/** @defgroup rng_file RNG
+
+@ingroup STM32F2xx
+
+@brief <b>libopencm3 STM32F2xx RNG</b>
+
+*/
+
+/*
+ * This file is part of the libopencm3 project.
+ *
+ * This library is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <libopencm3/stm32/rng.h>