cros_update -- filter out unstable packages.
cros_update -- filter out unstable packages.

For now, just look for x86. Also, provide an --upstream option so that we don't
have the checkout the gentoo branch if it's already available somewhere. This
allows faster testing (because dependencies don't need to be recalculated).

Output for libxml2 looks like this now:

[portage] dev-libs/libxml2-2.7.3-r2 -> dev-libs/libxml2-2.7.8
[portage] app-admin/eselect-python-20091230 -> app-admin/eselect-python-20100321
[portage] sys-libs/readline-6.0_p4 -> sys-libs/readline-6.1_p2
[chromiumos-overlay] sys-devel/libtool-2.2.6b -> sys-devel/libtool-2.2.10
[chromiumos-overlay] sys-devel/automake-1.11.1
[portage] sys-libs/zlib-1.2.3-r1 -> sys-libs/zlib-1.2.5-r2
[portage] app-arch/xz-utils-4.999.9_beta
[chromiumos-overlay] app-shells/bash-4.0_p35-r2 -> app-shells/bash-4.1_p9
[portage] sys-libs/ncurses-5.7-r3
[portage] sys-devel/automake-wrapper-5

BUG=chromium-os:13508
TEST=ran it

Change-Id: Iee33cf58f99109a5a149484de5ad5789af51ff49

Review URL: http://codereview.chromium.org/6758012

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,zhang0137/chromite