kernel: Fix symlinks

kernel: Fix symlinks

When installing a kernel package, the symlinks created by
update-alternatives should point to a path relative to KERNEL_IMAGEDEST
and not an absolute path to '/boot'.

Failing to do so causes problems when resolving the symlink inside some
bootloaders which mount the boot partition elsewhere.

Signed-off-by: David Vincent <freesilicon@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>