Linux target support

[AllianceTrooper]

Hello, RudolphRiedel.

I added Linux target support to the library. It uses Linux SPI API and libgpiod library.
Tested with Linux-running RPi 4B and NHD-3.5-320240FT-CSXP-T display.
Code: 5.x...AllianceTrooper:FT800-FT813:5.x
Does it make sense to merge it in upstream?
Thanks.

[RudolphRiedel]

Hello, by coincidence I do have a RPi2 here.
But I have no idea on how to actually use it, I merely tried to use it as a HDMI video source because it was lying around in the office.
And the other thing the prevented me from implementing RPi support or Linux support in general, I feel that this is way out of scope for the EVE series, the RPi do not only have HDMI output, but also MIDI-DSI support.
Well, obviously you felt a need for it and went for it, great, the more platforms the better. :-)

The only thing I noticed is that you are using "int", I try to avoid that in favour of explicit types.
But given the scope, not an issue at all.

So, yes, absolutely, if you like to, please put in a merge-request.
And if you like to, please also check out https://github.com/RudolphRiedel/EmbeddedVideoEngine which is the next version of this library that also supports BT820.

The changes to the target header files are minimal, there is one new function:
static inline void spi_transmit_32_addr(uint32_t data)