The USB code as it stands includes the bus glue along _with_ the controller
code. So the ohci/ehci modules actually build the USB controller code and
the PCI bus glue.
It'd be nice to ship separate modules for the PCI glue and the USB
controller (so for example if there were a USB controller hanging off
the internal SoC bus as well as an external PCI device) it could be done.
This is primarily done to save a few bytes here and there on embedded
systems with limited flash space for kernels - a very limited (sub-1MB)
space may be available for the kernel and may only support gzip encoding.
The rootfs can be LZMA compressed.
