x86 demo OS: ONIE interface implementation

To demonstrate how a NOS communicates with ONIE via the ONIE-CONFIG
partition the demo OS implements a number of CLIs:

install   -- boot ONIE in installer mode
rescue    -- boot ONIE in rescue mode
uninstall -- boot ONIE in OS uninstall mode
update    -- boot ONIE in self update mode

The implementation here highlights how an NOS would:

1. setup the onie-tools
2. use the onie-tools to change the boot order

Reviewed-By: dustin