Moved default AUDIO=en behavior to Makefile instead of build.sh

Makefile

@@ -195,6 +195,8 @@ $(error Invalid target: T=$(T))
 endif
 export T
 
+# Default audio source: en (English, config/_default_cfg_src_/res/en)
+AUDIO ?= en
 # Select audio source
 AUDIO_FOLDER = config/_default_cfg_src_/res/$(AUDIO)
 ifeq ($(AUDIO),)

build.sh

@@ -1,8 +1,5 @@
 #!/bin/bash
 
-# use AUDIO environment variable, default to "en"
-AUDIO="${AUDIO:-en}"
-
 make -j T=open_source DEBUG=1 >log.txt 2>&1
 
 if [ $? -eq 0 ]; then