cpu/esp32/esp-idf: minimum gpio definition for ESP-IDF headers

A number of ESP-IDF header files that are needed to compile RIOT include the header file `driver/gpio.h` only because of the definition of the type `gpio_num_t`. However, this header file contains the entire GPIO API definition of the ESP-IDF, which conflicts with that of RIOT.
The solution was to use a wrapper library that does not need to include the `driver/gpio.h` file of the ESP-IDF during compilation of RIOT code.
This commit provides another approach which does not require such a wrapper library. It just provides its own `driver/gpio.h` in RIOT that is included by ESP-IDF header files instead of the original `driver/gpio.h` in ESP-IDF. It  defines only the required `gpio_num_t` if RIOT code is compiled but includes the original `driver/gpio.h` of ESP-IDF if ESP-IDF code is compiled. This avoids to create a wrapper library for each module.

cpu/esp32/esp-idf/esp_idf.mk

@@ -1,6 +1,6 @@
 # common definitions for all ESP-IDF modules
 
-# additional include pathes required by als ESP-IDF module
+# additional include pathes required by ESP-IDF module
 INCLUDES += -I$(ESP32_SDK_DIR)/components/bootloader_support/include
 INCLUDES += -I$(ESP32_SDK_DIR)/components/bootloader_support/include_bootloader
 INCLUDES += -I$(ESP32_SDK_DIR)/components/driver/$(CPU_FAM)/include

cpu/esp32/esp-idf/esp_idf_cflags.mk

@@ -1,5 +1,8 @@
 # common definitions for all ESP-IDF modules
 
+# indicate that ESP-IDF code is compiled
+CFLAGS += -DESP_IDF_CODE
+
 # shortcuts used by ESP-IDF
 CFLAGS += -Dasm=__asm
 CFLAGS += -Dtypeof=__typeof__

cpu/esp32/esp-idf/include/driver/gpio.h

@@ -0,0 +1,24 @@
+#ifndef DRIVER_GPIO_H
+#define DRIVER_GPIO_H
+
+#ifdef ESP_IDF_CODE
+
+#include_next "driver/gpio.h"
+
+#else
+
+#include "hal/gpio_types.h"
+
+#define GPIO_PIN_COUNT  (SOC_GPIO_PIN_COUNT)
+
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* DRIVER_GPIO_H */