fix(sys-libs/glibc): Build only en_US locale by default.

This is only applicable to building the SDK (boards ship with only the
built-in C and POSIX locales). The SDK chroot sets en_US by default so
there is no reason to waste time building other locales for the tarball.

sdk_container/src/third_party/coreos-overlay/sys-libs/glibc/files/locale-default-en_US.patch

@@ -0,0 +1,13 @@
+--- extra/locale/locale.gen.orig	2014-03-18 10:38:23.993988978 -0700
++++ extra/locale/locale.gen	2014-03-18 10:38:38.140049907 -0700
+@@ -15,8 +15,8 @@
+ # rebuilt for you.  After updating this file, you can simply run `locale-gen`
+ # yourself instead of re-emerging glibc.
+ 
+-#en_US ISO-8859-1
+-#en_US.UTF-8 UTF-8
++en_US ISO-8859-1
++en_US.UTF-8 UTF-8
+ #ja_JP.EUC-JP EUC-JP
+ #ja_JP.UTF-8 UTF-8
+ #ja_JP EUC-JP

sdk_container/src/third_party/coreos-overlay/sys-libs/glibc/glibc-2.17.ebuild

@@ -156,6 +156,9 @@ eblit-src_unpack-pre() {
 }
 
 eblit-src_unpack-post() {
+	cd "${WORKDIR}"
+	epatch "${FILESDIR}"/locale-default-en_US.patch
+
 	if use hardened ; then
 		cd "${S}"
 		einfo "Patching to get working PIE binaries on PIE (hardened) platforms"