Fix a number of top level doxygen issues.

So that the navigation pane works correctly in browsers.
Some additional doc fixes put in where found (but many more still to go).
Added some dummy .c and .h files to bring the associated docs into line.
makefile changed to allow 'make html' as well as 'make doc' (the latter only does html anyway).

lib/stm32/f2/i2c.c

@@ -1,15 +1,15 @@
 /** @defgroup i2c_file I2C
-
-@ingroup STM32F2xx
-
-@brief <b>libopencm3 STM32F2xx I2C</b>
-
-@version 1.0.0
-
-@date 15 October 2012
-
-LGPL License Terms @ref lgpl_license
-*/
+ *
+ * @ingroup STM32F2xx
+ *
+ * @brief <b>libopencm3 STM32F2xx I2C</b>
+ *
+ * @version 1.0.0
+ *
+ * @date 15 October 2012
+ *
+ * LGPL License Terms @ref lgpl_license
+ */
 
 /*
  * This file is part of the libopencm3 project.
@@ -29,5 +29,5 @@ LGPL License Terms @ref lgpl_license
  */
 
 #include <libopencm3/stm32/i2c.h>
-#include <libopencm3/stm32/common/spi_common_all.h>
+#include <libopencm3/stm32/common/i2c_common_all.h>