[efi] Allow for optional protocols

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

Some UEFI systems (observed with a Mac Pro) do not provide
EFI_DEVICE_PATH_TO_TEXT_PROTOCOL.  Since we use this protocol only for
debug messages, make it optional and fall back to printing the raw
device path bytes.

Reported-by: Matt Woodward <pxematt@woodwardcc.com>
Tested-by: Matt Woodward <pxematt@woodwardcc.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Make EFI_HII_DATABASE_PROTOCOL optional

Some UEFI systems (observed with a Mac Pro) do not provide
EFI_HII_DATABASE_PROTOCOL.  We can continue to function without
providing access to network device settings via HII, so make this
protocol optional and fall back to simply not providing any HII
protocols.

Reported-by: Matt Woodward <pxematt@woodwardcc.com>
Tested-by: Matt Woodward <pxematt@woodwardcc.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Do not try to fetch loaded image device path protocol

Some UEFI systems (observed with a Mac Pro) do not provide a loaded
image device path protocol.  We don't currently use the loaded image
device path protocol for anything beyond printing a debug message, so
simply remove the code which attempts to fetch it.

Reported-by: Matt Woodward <pxematt@woodwardcc.com>
Tested-by: Matt Woodward <pxematt@woodwardcc.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>