scripts/nfs: fix nfs mount check for possible init symlink

initramfs-tools could just use a check like

chroot /root test -x /sbin/init

Then it doesn't matter whether it's a relative or absolute symlink.

Closes: #750360

Tested-by: Dominik George <nik@naturalnet.de>
Suggested-by: Michael Biebl <biebl@debian.org>
Signed-off-by: maximilian attems <maks@debian.org>