[settings] Expose PCI configuration space via settings mechanism

[settings] Expose PCI configuration space via settings mechanism

Allow values to be read from PCI configuration space using the syntax

  ${pci/<busdevfn>.<offset>.<length>}

where <busdevfn> is the bus:dev.fn address of the PCI device
(expressed as a single integer, as returned by ${net0/busloc}),
<offset> is the offset within PCI configuration space, and <length> is
the length within PCI configuration space.

Values are returned in reverse byte order, since PCI configuration
space is little-endian by definition.

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