doc: efm32: burtc: tag properly for doxyen

includes a stub .c file until there are APIs defined.

lib/efm32/common/burtc_common.c

@@ -0,0 +1,15 @@
+/** @addtogroup burtc_file BURTC peripheral API
+ * @ingroup peripheral_apis
+ * @brief Backup RTC helper functions.
+ *
+ * <b>NO</b> helper functions exist. Only header definitions are available.
+ * Delete these lines if/when you add actual helper APIs.
+ * @copyright See @ref lgpl_license
+ */
+
+#include <libopencm3/efm32/burtc.h>
+
+/**@{*/
+
+/**@}*/
+

lib/efm32/ezr32wg/Makefile

@@ -39,6 +39,7 @@ ARFLAGS		= rcs
 
 OBJS += acmp_common.o
 OBJS += adc_common.o
+OBJS += burtc_common.o
 OBJS += cmu_common.o
 OBJS += dac_common.o
 OBJS += dma_common.o

lib/efm32/lg/Makefile

@@ -39,6 +39,7 @@ ARFLAGS		= rcs
 
 OBJS += acmp_common.o
 OBJS += adc_common.o
+OBJS += burtc_common.o
 OBJS += cmu_common.o
 OBJS += dac_common.o
 OBJS += dma_common.o

lib/efm32/wg/Makefile

@@ -39,6 +39,7 @@ ARFLAGS		= rcs
 
 OBJS += acmp_common.o
 OBJS += adc_common.o
+OBJS += burtc_common.o
 OBJS += cmu_common.o
 OBJS += dac_common.o
 OBJS += dma_common.o