Add opus entries in generic makefiles.

libopus is necessary for opus software decoding support. add that to
base.mk and add an entry for libstagefright_soft_opusdec to
core_base.mk.

Change-Id: Iddd6ab2b2a84bb966b7e635a6aa73efb6a803e98
diff --git a/target/product/core_base.mk b/target/product/core_base.mk
index f14e53d..6c29482 100644
--- a/target/product/core_base.mk
+++ b/target/product/core_base.mk
@@ -50,6 +50,7 @@
     libstagefright_soft_mp3dec \
     libstagefright_soft_mpeg4dec \
     libstagefright_soft_mpeg4enc \
+    libstagefright_soft_opusdec \
     libstagefright_soft_rawdec \
     libstagefright_soft_vorbisdec \
     libstagefright_soft_vpxdec \