Replace and move ECNS configuration flag

ECNS configuration flag is modified in the offical code
so replace with the one landed in the branch. In addition, move
the configuration to the common makefile so that it applies to
all the target.

Change-Id: I86983c6605c6442228781cda36f51fded27bdcfd

b2g_product.mk

@@ -103,6 +103,9 @@ GAIA_DEFAULT_LOCALE=en-US
 MOZ_CHROME_MULTILOCALE=cs de hr hu pl sr ro
 endif
 
+# Target-specific Gecko configuration
+# Enable DSP-based EC/NS for webRTC
+GECKO_CONFIGURE_ARGS+=--enable-hardware-aec-ns
 
 # Env variables used for Gaia multilocale support
 LOCALE_BASEDIR=$(abspath gaia-l10n)