fix(profiles): Add shadow to the bootstrap package list

Despite the big scary warning saying otherwise upstream Gentoo has
actually included shadow in this list since 2011 (while also forgetting
to delete the warning, cute!). Not having it here runs the risk of
causing a failure during catalyst builds if something tries to add a
system user or group before shadow gets installed.

Upstream commit:
http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/profiles/default/linux/packages.build?r1=1.5&r2=1.6

sdk_container/src/third_party/coreos-overlay/profiles/default/linux/packages.build

@@ -5,9 +5,6 @@
 # This file describes the packages needed to build a stage1 based on this
 # profile.  Packages in this file are built in order.
 
-# While shadow could be in here, it breaks stage 1 and stage 2 building for the
-# releases.  PLEASE DO NOT ENABLE THIS UNLESS YOU HAVE THOROUGHLY TESTED IT!
-
 app-arch/bzip2
 app-arch/gzip
 app-arch/tar
@@ -27,9 +24,10 @@ sys-apps/gawk
 sys-apps/grep
 sys-apps/less
 sys-apps/net-tools
-sys-apps/sed
-sys-apps/texinfo
 sys-apps/portage
+sys-apps/sed
+sys-apps/shadow
+sys-apps/texinfo
 sys-devel/binutils
 sys-devel/bison
 sys-devel/flex