discover: Fix raw string argument to talloc_asprintf_append

discover: Fix raw string argument to talloc_asprintf_append

Current builds give a warning:

  ../discover/platform-powerpc.c: In function ‘update_bootdev_config’:
  ../discover/platform-powerpc.c:667:4: warning: format not a string
  literal and no format arguments [-Wformat-security]
      tmp = val = talloc_asprintf_append(val, boot_str);
          ^

This change uses "%s" for the format string.

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