[linux] Add support for accessing PCI configuration space via /proc/bus/pci

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[settings] Add config/settings.h

Move VMWARE_SETTINGS build configuration option from config/sideband.h
to a new config/settings.h.

Existing instances of config/local/sideband.h will not be affected,
since config.c still #includes config/sideband.h.

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