Symlink /{bin,sbin,lib64} to /usr/.

This also introduces a new USE flag, "symlink-usr".
Merge pull request #263 from oremj/symlink_usr

Symlink /{bin,sbin,lib64} to /usr/.
bump(sys-kernel/dracut): Update to dracut 034-r1

Copy from upstream, minus the sysvinit dependency.
fix(profiles): Enable dracut systemd module
fix(sys-apps/systemd-sysv-utils): Change install location, not target.

Change the install location based on symlink-usr instead of the target
and use absolute symlinks. We need to move towards never installing to
/{bin,sbin,lib...} and the use of absolute links avoids needlessly
breaking if the symlink-usr flag and the current state of /bin don't
agree (i.e. between flipping the flag and migrating the current files).
Fix ebuilds which conflict with symlink-usr.

This pulls in gawk, shadow, and coreutils. These ebuilds created
symlinks in /usr/bin which clobbered the binaries already in /usr/bin.
Merge pull request #265 from oremj/fix_usr_symlinks

Fix ebuilds which conflict with symlink-usr.
fix(coreos-base/coreos): Add kexec to base image.
Merge pull request #266 from marineam/kexec

fix(coreos-base/coreos): Add kexec to base image.
bump(sys-kernel/bootengine): Include journald config module
Merge pull request #264 from marineam/dracut

Dracut
fix(coreos-base/coreos-base): Remove nsswitch.conf

Provided by glibc now that it is being installed properly.
fix(profiles): Move default INSTALL_MASK to profiles.

The INSTALL_MASK is altered all over the place, clean it up by moving
all of it to profiles. Add /usr/share/{i18n,locales} to exclude those
installed by glibc since it doesn't have a nls use flag to disable them.
add(coreos-base/oem-gce): Add support for ssh keys on GCE
fix(profiles): Move INSTALL_MASK from common.sh to profiles.

This one isn't used by default, only when building final images like so:

    export INSTALL_MASK=$(portageq-$BOARD envvar PROD_INSTALL_MASK)
Merge pull request #267 from marineam/turtle-chain

Turtle chain
fix(coreos-base/oem-gce): Drop debug echo
Merge pull request #269 from marineam/gce

add(coreos-base/oem-gce): Add support for ssh keys on GCE
feat(coreos/base): accept keywords on docker 0.7

we want docker 0.7, accept it's unstable nature
feat(coreos/base): add lvm thin