tests: don't create a syslinux stub unconditionally in libtest

libtest.sh has an setup_os_repository() helper function tha is
used by many tests to setup an OSTree initial repository.

This function creates an syslinux configuration unconditionally
but OSTree supports other bootloader backends besides syslinux.

So, is better to conditionally create a syslinux configuration
only when it is needed.

https://bugzilla.gnome.org/show_bug.cgi?id=708069

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
tests: allow setup_os_repository to create U-Boot stub config

Currently OSTree supports two bootloader backends: syslinux
and U-Boot; allow to create a stub configuration for both.

https://bugzilla.gnome.org/show_bug.cgi?id=708069

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
admin: add U-Boot backend test case

OSTree now supports a backend for the U-Boot bootloader,
add a test case for this.

https://bugzilla.gnome.org/show_bug.cgi?id=708069

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>