Removing etc1 from build for javascript platform.

Based on inputs from reduz
1) Made etc1 optional driver and default yes.
2) Reverted the hack in rg_etc1.cpp
3) Disabled etc1 for javascript plaform.

SConstruct

@@ -111,6 +111,7 @@ opts.Add('jpg','JPG Image loader support (yes/no)','yes')
 opts.Add('webp','WEBP Image loader support (yes/no)','yes')
 opts.Add('dds','DDS Texture loader support (yes/no)','yes')
 opts.Add('pvr','PVR (PowerVR) Texture loader support (yes/no)','yes')
+opts.Add('etc1','etc1 Texture compression support (yes/no)','yes')
 opts.Add('builtin_zlib','Use built-in zlib (yes/no)','yes')
 opts.Add('openssl','Use OpenSSL (yes/no/builtin)','no')
 opts.Add('musepack','Musepack Audio (yes/no)','yes')