feat(release_util): Add support for signing individual file uploads.

The .DIGESTS format is clunky and annoying. It also requires uses to
perform two steps to verify images using GPG. Instead support signing
all files directly so there is no need for .DIGESTS.

The old DIGESTS code will remain in place for now but after a few
releases I plan on deleting it.
Merge pull request #230 from marineam/gpg

feat(release_util): Add support for signing individual file uploads.
fix(oem/ami): Disable parallel AMI copying

The parallelism appears to be what was breaking the fussy ec2 command
line tools. Just do it serially instead.
Merge pull request #231 from marineam/ami

fix(oem/ami): Disable parallel AMI copying
maint(toolchain_util): Drop amd64-generic from list of valid boards.

The cleanup begins...
maint(*): Delete special case code for amd64-generic
Merge pull request #232 from marineam/purge

Purge amd64-generic
fix(disk_util): Switch back to gptmbr.bin for our MBR boot code.

Using the classic mbr.bin was only needed during the transition from
syslinux 3 to 6 because the behavior of gptmbr.bin changed after 3.
Now that the transition is done and cgpt supports the new scheme now it
is time we switched back. This avoids depending on using a hybrid MBR.
fix(syslinux): Display boot: prompt for a half second.

This should make it less difficult for people to add kernel options for
debugging. Without a prompt/timeout the user must be holding down space
or some other key while syslinux loads but it may not be possible for
the user to do so provide input quite that fast. Only a half second to
avoid needlessly increasing boot times in the common case.
Merge pull request #234 from marineam/syslinux

fix(syslinux): Display boot: prompt for a half second.
Merge pull request #233 from marineam/mbr

fix(disk_util): Switch back to gptmbr.bin for our MBR boot code.
maint(build_library): Remove special case code for -usr disk layouts

More cleanup now that amd64-generic is gone.
Merge pull request #235 from marineam/clean

maint(build_library): Remove special case code for -usr disk layouts
feat(vm_image_util): Add new build steps, update vagrant builds

This adds two new optional build steps.  The first user of these is the
vagrant images but many of the targets can be simplified now.

- fs_hook: Anything that needs to happen before unmounting the image.
  This happens after the OEM is installed but before disk images are
  made. It can be used to copy any data out of the image.
- bundle_format: Many VM types ship as some sort of archive format
  rather than plain disk images as this script originally assumed.
  Adding this final step lets us stop using the conf step awkwardly.

Vagrant now ships with a Vagrantfile and related code included in the
OEM package. This lets us version our vagrant-side code along with the
images themselves as well making the coreos-vagrant repo optional again.
The coreos-vagrant code will still be useful for handling the fancier
cluster configuration stuff but no longer has to carry the plugin code.
feat(vm_image_util): Add Vagrant metadata file

This is required for Vagrant to recognize box versions:
http://docs.vagrantup.com/v2/boxes/versioning.html
maint(set_lsb_release): Remove gentoo-release (again)

We now require Vagrant 1.5.0 both in documentation and in code which
uses os-release instead of gentoo-release.
fix(common): Switch to os-release style names in version.txt

The existing version.txt is kinda annoying. The common case of referring
to the current version requires joining three values and the names of
those values only make sense in ChromeOS. Instead just use version as a
string, using VERSION, VERSION_ID, and BUILD_ID just as they appear in
os-release. It is up to the few scripts that need the individual parts
to break the version apart.

The old values remain for the sake of compatibility.
fix(vm_image_util): Fix building vagrant vmware fusion images.
fix(prod_image_util): Do not install update test key anymore.

Leave this to the ebuild, no reason for it to be done here.
feat(disk_util): Add support for extracting partitions.

This will simplify the process of generating update payloads.