[build] Construct driver lists for each bus type

[build] Construct driver lists for each bus type

Include the underlying bus type (e.g. "pci" or "isa") within the lists
constructed to describe the available drivers, to allow for the
possibility that platforms may want to define a platform-specific
subset of drivers to be present in the all-drivers build.  For
example, non-x86 platforms such as RISC-V SBI do not need to include
the ISA network drivers since the corresponding hardware cannot ever
be present on a RISC-V system.

Signed-off-by: Michael Brown <mcb30@ipxe.org>