fix(common): Read mounts from /proc/self/mounts

Just in case the filesystem view is slightly different.
fix(common): Fix okboat and failboat because boat.
fix(common): Simplify unmount code, die loud and die hard.

As-is safe_umount is extremely dangerous. When passed multiple mount
points and any one of them fail with a "not mounted" or "doesn't exist"
error then any others that fail with a more serious error will be
silently ignored. This can cause untold sadness when running deleting a
chroot with cros_sdk if /mnt/host/source is left mounted, all your code
will be gone. To avoid this situation remove *ALL* this extra logic and
die very loudly when umount fails. Due to the way bind mounts interact
with this code "not mounted" so when unmounting a full tree we need to
still need to gracefully retry when the first umount fails.
fix(build_image): Don't unmount when the rootfs isn't mounted

This function is never called when rootfs is mounted but leaving in a
check for it as a just in case sort of thing.
Merge pull request #44 from marineam/fix-umount

Fix umount
feat(build_image): install the au-key

Install the developer au key on production images. This will need to
change when we have the production key.
fix(cros_make_image_bootable): no more bootkernel

the bootkernel is going away and we will just have a single kernel for
all of the things. This is because of the kxec on xen problem.
Merge pull request #45 from philips/switchup-bootkernel

Switchup bootkernel
fix(cros_make_image_bootable): need to mkdir first

The au directory doesn't exist. Make the directory first.
Merge pull request #46 from philips/fix-au-key

fix(cros_make_image_bootable): need to mkdir first
feat(build_image): official lsb-release on prod images

The /etc/lsb-release should have official values on the prod image. That
way someone can ping the official api.core-os.net with this image.
Merge pull request #47 from philips/make-lsb-release-official-on-prod

feat(build_image): official lsb-release on prod images
feat(build_library): reintroduce A/B menu.lst

reintroduce unique A/B menu.lsts to work around the kexec problems that
we have. Essentially instead of always using boot_kernel on pvgrub
systems use the A/B kernels installed at update time to the boot
partition.
Merge pull request #48 from philips/reintroduce-unique-a-b-menu.lst

feat(build_library): reintroduce A/B menu.lst
add rackspace