Having a default script containing

  #!gpxe
  autoboot

can cause problems when entering commands to load and start a kernel
manually; the default script image will still be present when the
kernel is started and so will be treated as an initrd.  It is possible
to work around this by typing "imgfree" before any other commands, but
this is counter-intuitive.

Fix by allowing the embedded image list to be empty (in which case we
just call autoboot()), and making this the default.

Reported by alkisg@gmail.com.

The "chain" command combines the functions of "imgfetch", "imgload",
and "boot".

    chain http://etherboot.org/gtest/gtest.gpxe

is equivalent to:

    kernel http://etherboot.org/gtest/gtest.gpxe
    boot

If there is more than one loaded image, refuse to automatically select
the image to execute.  There are at least two possible cases, with
different "correct" answers:

1. User loads image A by mistake, then loads image B and types "boot".
   User wants to execute image B.

2. User loads image A, then loads image B (which patches image A), then
   types "boot".  User wants to execute image A.

If a user actually wants to load multiple images, they must explicitly
specify which image is to be executed.

We can just treat all non-kernel images as initrds, which matches our
behaviour for multiboot kernels.  This allows us to eliminate initrd as
an image type, and treat the "initrd" command as just another synonym for
"imgfetch".

This fixes a bug introduced in commit 4c85017.

Some shuffling around of the image management code; this needs tidying up.

"kernel".