[efi] Avoid returning uninitialised data from PCI configuration space reads

Under UEFI, reads from PCI configuration space may fail.  If this
happens, we should return all-ones (which will mimic the behaviour of
an absent PCI device).

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Make EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL optional

Some UEFI systems (observed with a Hyper-V virtual machine) do not
provide EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL.  Make this an optional
protocol (and fail any attempts to access PCI configuration space via
the root bridge if the protocol is missing).

Reported-by: Colin Blacker <Colin.Blacker@computerplanet.co.uk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Allow for non-PCI snpnet devices

We currently require information about the underlying PCI device to
populate the snpnet device's name and description.  If the underlying
device is not a PCI device, this will fail and prevent the device from
being registered.

Fix by falling back to populating the device description with
information based on the EFI handle, if no PCI device information is
available.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Clean up all binary directories on "make [very]clean"

Allow a straightforward "make clean" or "make veryclean" to apply to
all binary directories (using the shell pattern "bin{,-*}").
Individual binary directories can be cleaned using e.g.

  make bin clean
  make bin-x86_64-efi clean

Reported-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>