This patch extends the embedded image feature to allow multiple
embedded images instead of just one.

gPXE now always boots the first embedded image on startup instead of
doing the hardcoded DHCP boot (aka autoboot).

Based heavily upon a patch by Stefan Hajnoczi <stefanha@gmail.com>.

OpenBSD throws compiler warnings that we can't reproduce on Linux, for
some reason.

Original patch from Dewey Hylton <dewey@hyltown.com>.

When the embedded image is a script, the unregister_image() performed by
image/script.c corrupts memory, since image/embedded.c omitted the call
to register_image().

This is the first bug fixed using Stefan Hajnoczi's gdb stub for gPXE.