Add `sysroot.bootprefix` option

Add `sysroot.bootprefix` option

This is a follow up to
https://github.com/ostreedev/ostree/pull/2149/commits/0ced9fde7649271d9458ca424aa8c41908634b02
"sysroot: Support /boot on root or as seperate filesystem for syslinux and u-boot"

What we should have done at the time is changed our bootloader entries
to be prefixed with `/boot`.  This means that the GRUB2 BLS support
will Just Work.

For now, I'm making this option default to off out of a lot of
conservatism.  I think in the future we should flip this on by default.