Audio enable corrections (Part 4) (#8974)

* audio-configuration: throw a compile error on AUDIO_ENABLE=yes with no output pin configured

relevant for avr boards, arm currently implicitly uses A4/A5

* audio-configuration: add Changelog entry

* audio-configuration: remove stray newline

Author: JohSchneider

docs/ChangeLog/20200530/PR8974.md

@@ -0,0 +1,2 @@
+## fixing wrong configuration of AUDIO feature
+add a compile-time error to alert the user to a missing pin-configuration (on AVR boards) when AUDIO_ENABLE=yes is set

quantum/audio/audio_avr.c

@@ -110,6 +110,11 @@
 #    define TIMER_1_DUTY_CYCLE OCR1C
 #    define TIMER1_AUDIO_vect TIMER1_COMPC_vect
 #endif
+
+#if !defined(BPIN_AUDIO) && !defined(CPIN_AUDIO)
+#    error "Audio feature enabled, but no suitable pin selected - see docs/feature_audio.md under the AVR settings for available options."
+#endif
+
 // -----------------------------------------------------------------------------
 
 int   voices        = 0;