disk_layout: mount ESP to /boot instead of /boot/efi

We have long since stopped installing anything to the /boot directory of
the root filesystem. Mount the ESP partition to /boot for consistancy
with the discoverable partition spec.
disk_layout: replace unused "BOOT-B" with Grub's BIOS Boot Partition

Unlike SYSLINUX, GRUB2 does not recommend embedding itself in a FAT
filesystem. Instead GRUB2 prefers embedding in the space between the MBR
and first partition or using a dedicated partition that is safe from
tampering by fs utilities. In our case the space after the MBR is where
the GPT lives so we need to use the extra partition scheme instead.

The 64MB "BOOT-B" partition has never been used so we can replace it
with a 2MB partition which is more than enough for GRUB.
disk_util: do not zero MBR and GPT when resizing disk

Calling cgpt create when resizing zeros the MBR boot code. This worked
with the syslinux setup because the boot code was re-written. When not
using syslinux it is easier to just preserve the existing MBR instead.
disk_util: support exposing a hybrid partition without syslinux

We don't need to do anything like manually install the MBR boot code
for grub but we do need to continue to expose the ESP partition as a
hybrid partition to support pvgrub.
build_image_util: pass the disk image through to configure_bootloaders

Required so that configure_bootloaders can now handle installing the
bootloaders as well.
configure_bootloaders: add grub install stub code

Not currently used, this configuration which sets up grub to re-use the
syslinux configuration only works with recent git versions, not any
releases. Compatibility is also limited because the serial configuration
in syslinux must be duplicated in the grub config.
Merge pull request #324 from marineam/grub

Grub2 preview