Replace implicit boot from hdd with explicit via ipxe script

[mmlb]

There are times when we want to force a PXE booting machine to instead boot from disk. Think of machines that have netboot as the default boot option. We want to do this to avoid waiting for netboot
to timeout and proceed to boot from disk for each and every boot. We currently do this by sending a tftp file named /nonexiistent or /pxe-is-not-allowed (depending on if #149 is merged yet) but @splaspood mentioned that there's a way to boot from disk from withing iPXE.

Looking at netboot.xyz's iPXE script we see:

:local
echo Booting from local disks ...
exit 0

We should do the same instead of the non-existent tftp file. Doing so will extend the pxe boot process a little bit, especially after #149 is merged. This should be neglible.