Host preparation file writing is already done by the __init__ method
of iPXEBoot. This change moves place_loaders_for_boot calls to
iPXEBoot and PXEBoot to be consistent, and to only write the files
when that driver is enabled.
This will mean multiple writes of the same file when subclasses of
these drivers are also enabled, but this overhead will be negligible.
Change-Id: I7e17f4d1a54cd6c5d1a4bf006a0d42db8d123a46
45e8adc1df