ONIE boot mode behaviours: rescue and install

The powerpc version of ONIE has the following behaviours for rescue
and install mode.

This patch adds changes to the generic boot time framework and
architecture specific implementations for powerpc and x86_64.

Rescue Mode
===========

We want rescue mode booting to be a one shot operation.
After the rescue mode we should reboot into the previous default boot
entry.  How to do that is an architecture specific detail.

An architecture must provide an override of the
rescue_revert_default_arch() function in discover.sh

Install Mode
============
We want install mode booting to be sticky, i.e. if you boot into
install mode you stay install mode until an installer runs
successfully.  How to do that is an architecture specific detail.

An architecture must provide an override of the
install_remain_sticky_arch() functions in discover.sh

Testing:

Rescue Mode -- On both a powerpc and x86_64 platforms performed the
following test:

1. install the demo OS
2. from the demo OS enter rescue mode with the 'rescue' command.
3. the box reboots into ONIE resuce mode.
4. reboot the box
5. verify the box returns to the Demo OS.

That concludes the 'rescue mode' one-shot behaviour test.

Install Mode -- On both a powerpc and x86_64 platforms performed the
following test:

1. install the demo OS
2. from the demo OS enter install mode with the 'install' command.
3. the box reboots into ONIE install mode.
4. before an install happens, reboot the box.
5. verify the box returns to ONIE install mode.

That concludes the 'install mode' sticky behaviour test.

Reviewed-By: dustin