drivers/periph: use uint_fast8_t as default type

The default driver type is just an index into a device array defined
by the board.

If a platform wants to encode additional information in the device type,
it can define a custom type.

This means we can just set the default type to whatever fits the target
CPU best.

On ARM this will still be a 32 bit word, but on AVR it will by a 8 bit byte.

drivers/include/periph/adc.h

@@ -69,14 +69,14 @@ extern "C" {
  * @brief   Define default ADC type identifier
  */
 #ifndef HAVE_ADC_T
-typedef unsigned int adc_t;
+typedef uint_fast8_t adc_t;
 #endif
 
 /**
  * @brief   Default ADC undefined value
  */
 #ifndef ADC_UNDEF
-#define ADC_UNDEF           (UINT_MAX)
+#define ADC_UNDEF           (UINT_FAST8_MAX)
 #endif
 
 /**

drivers/include/periph/dac.h

@@ -64,7 +64,7 @@ extern "C" {
  * @brief   Define default DAC type identifier
  */
 #ifndef HAVE_DAC_T
-typedef unsigned int dac_t;
+typedef uint_fast8_t dac_t;
 #endif
 
 /**
@@ -79,7 +79,7 @@ enum {
  * @brief   Default DAC undefined value
  */
 #ifndef DAC_UNDEF
-#define DAC_UNDEF           (UINT_MAX)
+#define DAC_UNDEF           (UINT_FAST8_MAX)
 #endif
 
 /**

drivers/include/periph/i2c.h

@@ -139,7 +139,7 @@ extern "C" {
  * @{
  */
 #ifndef I2C_UNDEF
-#define I2C_UNDEF           (UINT_MAX)
+#define I2C_UNDEF           (UINT_FAST8_MAX)
 #endif
 /** @} */
 
@@ -148,7 +148,7 @@ extern "C" {
  * @{
  */
 #ifndef HAVE_I2C_T
-typedef unsigned int i2c_t;
+typedef uint_fast8_t i2c_t;
 #endif
 /**  @} */
 

drivers/include/periph/pwm.h

@@ -82,14 +82,14 @@ extern "C" {
  * @brief  Default PWM undefined value
  */
 #ifndef PWM_UNDEF
-#define PWM_UNDEF           (UINT_MAX)
+#define PWM_UNDEF           (UINT_FAST8_MAX)
 #endif
 
 /**
  * @brief   Default PWM type definition
  */
 #ifndef HAVE_PWM_T
-typedef unsigned int pwm_t;
+typedef uint_fast8_t pwm_t;
 #endif
 
 /**

drivers/include/periph/qdec.h

@@ -98,14 +98,14 @@ extern "C" {
  * @brief  Default QDEC undefined value
  */
 #ifndef QDEC_UNDEF
-#define QDEC_UNDEF           (UINT_MAX)
+#define QDEC_UNDEF           (UINT_FAST8_MAX)
 #endif
 
 /**
  * @brief   Default QDEC type definition
  */
 #ifndef HAVE_QDEC_T
-typedef unsigned int qdec_t;
+typedef uint_fast8_t qdec_t;
 #endif
 
 /**

drivers/include/periph/spi.h

@@ -91,7 +91,7 @@ extern "C" {
  * @brief   Define global value for undefined SPI device
  */
 #ifndef SPI_UNDEF
-#define SPI_UNDEF       (UINT_MAX)
+#define SPI_UNDEF       (UINT_FAST8_MAX)
 #endif
 
 /**
@@ -116,7 +116,7 @@ extern "C" {
  * @brief   Default type for SPI devices
  */
 #ifndef HAVE_SPI_T
-typedef unsigned int spi_t;
+typedef uint_fast8_t spi_t;
 #endif
 
 /**

drivers/include/periph/timer.h

@@ -56,7 +56,7 @@ extern "C" {
  * @brief   Default value for timer not defined
  */
 #ifndef TIMER_UNDEF
-#define TIMER_UNDEF         (UINT_MAX)
+#define TIMER_UNDEF         (UINT_FAST8_MAX)
 #endif
 
 /**
@@ -66,7 +66,7 @@ extern "C" {
  * and vendor device header.
  */
 #ifndef HAVE_TIMER_T
-typedef unsigned int tim_t;
+typedef uint_fast8_t tim_t;
 #endif
 
 /**

drivers/include/periph/uart.h

@@ -74,14 +74,14 @@ extern "C" {
  * @brief   Define default UART type identifier
  */
 #ifndef HAVE_UART_T
-typedef unsigned int uart_t;
+typedef uint_fast8_t uart_t;
 #endif
 
 /**
  * @brief   Default UART undefined value
  */
 #ifndef UART_UNDEF
-#define UART_UNDEF          (UINT_MAX)
+#define UART_UNDEF          (UINT_FAST8_MAX)
 #endif
 
 /**