Enable configuring target variants

This change enables picking the right board configuration from the
variants/ folder. Previously, we would always pick the default
configuration (e.g., "esp32" instead of "heltec_wifi_lora_32_V2").

CMakeLists.txt

@@ -168,7 +168,7 @@ set(BLE_SRCS
   )
 
 set(includedirs
-  variants/${IDF_TARGET}/
+  variants/${CONFIG_ARDUINO_VARIANT}/
   cores/esp32/
   libraries/ArduinoOTA/src
   libraries/AsyncUDP/src
@@ -210,13 +210,13 @@ set(priv_requires fatfs nvs_flash app_update spiffs bootloader_support openssl b
 
 idf_component_register(INCLUDE_DIRS ${includedirs} PRIV_INCLUDE_DIRS ${priv_includes} SRCS ${srcs} REQUIRES ${requires} PRIV_REQUIRES ${priv_requires})
 
-string(TOUPPER ${CONFIG_IDF_TARGET} idf_target_caps)
+string(TOUPPER ${CONFIG_ARDUINO_VARIANT} idf_target_caps)
 target_compile_options(${COMPONENT_TARGET} PUBLIC
     -DARDUINO=10812
     -DARDUINO_${idf_target_caps}_DEV
     -DARDUINO_ARCH_ESP32
     -DARDUINO_BOARD="${idf_target_caps}_DEV"
-    -DARDUINO_VARIANT="${CONFIG_IDF_TARGET}"
+    -DARDUINO_VARIANT="${CONFIG_ARDUINO_VARIANT}"
     -DESP32)
 
 if(CONFIG_AUTOSTART_ARDUINO)

Kconfig.projbuild

@@ -1,5 +1,14 @@
 menu "Arduino Configuration"
 
+config ARDUINO_VARIANT
+    string "Arduino target variant (board)"
+    default IDF_TARGET
+    help
+        The name of a target variant (e.g., a specific board) in the variants/
+        folder, e.g.  "heltec_wifi_lora_32_V2". The name is case sensitive.
+        Specifying a variant name different from the target enables additional
+        customization, for example the definition of GPIO pins.
+
 config ENABLE_ARDUINO_DEPENDS
     bool
     select LWIP_SO_RCVBUF