sysroot: Support /boot on root or as seperate filesystem for syslinux and u-boot

sysroot: Support /boot on root or as seperate filesystem for syslinux and u-boot

We use a similar trick to having a `sysroot -> .` symlink on the real root
here to support both /boot on root as well as on a separate filesystem.  No
matter how it's mounted `/boot/xyz` will always refer to the file you'd
expect.

This is nicer than my previous attempts at this because there's no
configuration nor auto-detection required.