boards/nucleo-f091: add i2c configuration

boards/nucleo-f091rc/Makefile.features

 # Put defined MCU peripherals here (in alphabetical order)
 FEATURES_PROVIDED += periph_adc
 FEATURES_PROVIDED += periph_gpio
+FEATURES_PROVIDED += periph_i2c
 FEATURES_PROVIDED += periph_pwm
 FEATURES_PROVIDED += periph_rtc
 FEATURES_PROVIDED += periph_timer

boards/nucleo-f091rc/include/periph_conf.h

@@ -159,6 +159,29 @@ static const spi_conf_t spi_config[] = {
 #define SPI_NUMOF           (sizeof(spi_config) / sizeof(spi_config[0]))
 /** @} */
 
+/**
+ * @name    I2C configuration
+ * @{
+ */
+static const i2c_conf_t i2c_config[] = {
+    {
+        .dev            = I2C1,
+        .speed          = I2C_SPEED_NORMAL,
+        .scl_pin        = GPIO_PIN(PORT_B, 8),
+        .sda_pin        = GPIO_PIN(PORT_B, 9),
+        .scl_af         = GPIO_AF1,
+        .sda_af         = GPIO_AF1,
+        .bus            = APB1,
+        .rcc_mask       = RCC_APB1ENR_I2C1EN,
+        .rcc_sw_mask    = RCC_CFGR3_I2C1SW,
+        .irqn           = I2C1_IRQn,
+    }
+};
+
+#define I2C_0_ISR           isr_i2c1
+
+#define I2C_NUMOF           (sizeof(i2c_config) / sizeof(i2c_config[0]))
+/** @} */
 
 /**
  * @name    PWM configuration