syslinux-install_update: do not fail if /boot/syslinux does not exist
requested to merge nl6720/package-syslinux:fix-syslinux-install_update-without-boot-syslinux into main
On a fresh install, there is no /boot/syslinux directory.
Fixes #3
Fixes: c0dbb98b ("Move syslinux.cfg to /usr/share/syslinux/")
/cc @h2odriving @eworm