add 2buffer transfer version

[jayzakk]

PR as of trying to get Ethernet.h faster.
See notes to Ethernet.h at https://github.com/dbuezas/lgt8fx/wiki/Libraries-support
The library uses a (then existant) function, if SPI_HAS_TRANSFER_BUF is defined. If not, it does call transfer() for each byte.

As of Teeny source:

// SPI_HAS_TRANSFER_BUF - is defined to signify that this library supports
// a version of transfer which allows you to pass in both TX and RX buffer
// pointers, either of which could be NULL

Added this define, and added the function to do so.
The function is not fast enough to keep SCK running free at 16MHz SPI, but still faster than the one used before.
Reading+Writing with 2MByte/sec at 32MHz MCU doesn't seem possible ^^

At 8MHz SPI it makes SOME difference.

[jayzakk]

I currently have some more optimizations, but sometimes the xfer stalls after some time.
For now, I GUESS it's the very long cables from SPI to W5500; 16MHz SPI is quite a lot.

IF it is just from the cables, I could get 10.7MBit/s (from 6.08 before) SEND and (after changing w5100.cpp to use this new function) 14.1MBit/s RECEIVE (from 8.2 before).

Will doublecheck/shorten the cables tomorrow.

[jayzakk]

So, it was the cable. 12cm flexwire is too much for 16MHz. 6cm does...

I refactored to move the inlined segment to the class, it got too big for inlining a couple of times.
Additional PR (arduino-libraries/Ethernet#145) to use the transfer-double-buffer-SPI also on receive, not only on send.

Works like a charm. Updated performance data at the libraries page.

[dbuezas]

Do you want to add an example in https://github.com/dbuezas/lgt8fx/tree/master/lgt8f/libraries/lgt328p/examples ? :)
I'm not sure if anybody will realise about the new method otherwise

[dbuezas]

I just realised the function you added is standard in other boards, so it shouldn't need examples.
Merging now