build: Tweaks to better support non-English as primary language

* get rid of $(LANGUAGE) in top-level makefile (and configure script)
 * un-hardcode English-as-primary-language in a couple more places
 * allow DEFAULT_VOICE_LANG to be overriden

To actually change the primary from English, one must change:

 * $english in voice.pl
 * hardcoded 'english' in rbutil
 * $ENGLISH in apps/lang/lang.make
 * DEFAULT_VOICE_LANG in apps/talk.c
 * configure script (default prompt)

Of course, if one wants to change the default UI language, it's simpler
to change the default language setting variable at compile time, or
perhaps by adding a configuration file with the desired value into the
.rockbox directory when the .zip is assembled.

Change-Id: If5cf76019d416e838628a2eccd4ec7d6cbaeeb74

tools/configure

@@ -4669,9 +4669,6 @@ fi
 
 echo "Using source code root directory: $rootdir"
 
-# this was once possible to change at build-time, but no more:
-language="english"
-
 uname=`uname`
 
 if [ "yes" = "$simulator" ]; then
@@ -5039,7 +5036,6 @@ export MANUFACTURER=${t_manufacturer}
 export OBJDIR=${pwd}
 export BUILDDIR=${pwd}
 export RBCODEC_BLD=${pwd}/lib/rbcodec
-export LANGUAGE=${language}
 export VOICELANGUAGE=${voicelanguage}
 export MEMORYSIZE=${memory}
 export BUILDDATE:=\$(shell date -u +'-DYEAR=%Y -DMONTH=%m -DDAY=%d')