kernel-install/90-loaderentry: fix when /boot is not mountpoint

I happen to have a machine where /boot is not a separate mountpoint,
but rather just a directory under /. After upgrade to recent Fedora,
I found out that grub2 can't find any new kernels.

This happens because loadentry script generates kernel and initrd file
paths relative to /boot, while grub2 expects path to be relative to the
root of filesystem on which they are residing.

This commit fixes this issue by using stat's %m to find the mount point
of a partition holding the images, and using it as a prefix to be
removed from ENTRY_DIR_ABS.

Note that %m for stat requires coreutils 8.6, released in Oct 2010.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>