fix(pvgrub): Disable support for gptprio root detection/fallback.

This scheme only works robustly with kexec. Until the happy day that
kexec is supported on Xen (or when Xen is dead, long live Xen!) we
shouldn't bother trying. This allows us to use kernel modules again.
fix(syslinux): Apply sudo_clobber fix to syslinux configs.

Already cleaned up the contrived cat/dd combo for the pvgrub configs, do
the same for the syslinux configs.
fix(pvgrub): Stick root's copy of the configs in /boot/grub

Previously this was in /boot/grub/grub which was probably by mistake.
cleanup(update_bootloaders.sh): Remove dead code.
Merge pull request #133 from marineam/modules

Modules n' Bootloaders