Move boot to discover server

This change moves the boot-via-kexec functionality from the UIs to the
discover server.

On the UI side: rather than run kexec directly, we just send a message
to the discover server. Because this is generic discover client
functionality, we no longer need the boot callbacks in the twin- and
ncurses-specific code.

We also remove the kexec and URL-loading code from the UIs, and add it
to the discover server code, in paths.c. We expose this to the server
though a new function:

  load_path(void *, const char *, unsigned int *);

On the server side, we simply move hook up the boot() function to use
the load_file and kexec calls.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>