Add boot module

[gurry]

Add a module called boot which exposes PXE and DHCP functions in an ergonomic way. It should provide the following functionality:

• Running DHCP
• Running PXE
• Accessing stored DHCP information, e.g. last DHCP Ack etc.
• Booting into a given image - would ideally be some trait like BootTarget

Once this is done we will hide/remove the current low-level PxeBaseCodeProtocol object we currently access through the boot services.