support for multiple kernel versions of the same flavour in live-bnuild generated syslinux menu

[Michal Suchanek <hramrach@gmail.com>]

Hello,

this patch

- automatically inserts any extra kernels you install in the syslinux menu
- automatically inserts any kernels built with kernel-packages from
git (flavour has extra + at the end)
- adds an option to specify different flavours for syslinux. This both
controls how the kernel appear in the menu and in what order

This appears to work reasonably well to go in the alpha version.

Most edge cases are untested.

issues:
- the auto/ scripts do not have access to the defaults. Specifically
the default flavour list to generate a modififed flavour list if
needed.
- the chroot hook scripts do not have access to configuration.
Specifically the flavour list is not available for a chroot hook  to
install extra kernels of all flavours.

Thanks

Michal