update-rc.d: check also that symlinks are valid

Running:

update-rc.d -r /path/to/target/rootfs basename defaults

at do_rootfs time in package postinstall stage, when
/path/to/target/rootfs/etc/init.d/basename is a symlink and points to some path
on target (for example: /etc/init.d/basename.some_package), would fail and the
postinstall execution would be postponed for first boot, on target.

This patch adds the posibility to verify whether the file the symlink
points to actually exists in the target rootfs.

[YOCTO #3716]

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>