x86 .iso installer: add 'rescue' and 'embed' menu entries

Reviewed By: tbd

The previous .iso image was rather spartan and booted straight into
ONIE 'rescue' mode.  This patch adds a proper syslinux menu with two
choices:

- rescue (the default)
- embed (installs the ONIE image contained in the .iso)

This image could easily be recompiled with the defaults swapped,
making an unassisted 'ONIE embed' USB memory stick suitable for
manufacturing.

Testing Done:

Compiled the .iso image and booted it using the -cdrom option to the
kvm virtual machine.  Verified that both menu items worked as
expected.