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/common/rng_common_v1.c

@@ -0,0 +1,44 @@
+/** @addtogroup rng_file
+ *
+ * This library supports the random number generator peripheral (RNG) in the
+ * STM32F4 series of ARM Cortex Microcontrollers by ST Microelectronics.
+ *
+ * LGPL License Terms @ref lgpl_license
+ */
+
+/*
+ * 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>
+
+/**@{*/
+
+/** Disable the Random Number Generator peripheral.
+*/
+void rng_disable(void)
+{
+    RNG_CR &= ~RNG_CR_RNGEN;
+}
+
+/** Enable the Random Number Generator peripheral.
+*/
+void rng_enable(void)
+{
+    RNG_CR |= RNG_CR_RNGEN;
+}
+
+/**@}*/