bootscript: update to account for u-boot changes

u-boot has been updated to use config_distro_defaults.h to enable easier
integration with future distros.
This also enables some extra features that require renaming some variables
to work properly.
add extlinux.conf

this makes use of some changes provided by config_distro_defaults.h
the most interesting of which is the sysboot command which allows us to
use a limited emulation of the syslinux menu system.
The big advantage here is that we can offer several kernels to boot from
in much the same way as you'd have multiple options with grub or lilo

This example file provides a simple default 'linux' boot and a 'recovery'
boot whuch curently just prefixes the kernel and devicetree files with .rec

You can include an 'append' option for kernel parameters, but note doing so
will override the u-boot bootargs variable and that you cannot use u-boot
shell variables in the append line which means you can't use the PARTUUID
trick to avoid needing to specify the root partition, you'll need to
hard code either the /dev name or the uuid into extlinux.conf if you
use append.