Move most flash code from pio to libpixi
libpixi: move flash functions into pixi_ namespace
libpixi: hide the flash SpiDevice
libpixi: document flash functions
libpixi: automatically set the application log level

Inherit libpixi log level, but set from {uppercase(appname)}_LOG_LEVEL
if that is found in the environment.
pio: remove pio_logLevel and use pixi_appLogLevel
libpixi: move command handling from pio to libpixi
pio: remove superfluous channel argument from PiXi SPI commands
pio: make better use of libpixi SPI functions in spi commands

Including using multiRegisterOp for spi-scan.
libpixi: fixes for command.h

Enable c++ support.
Add LIBPIXI_COMMAND_GROUP macro, instead of redefining LIBPIXI_CONSTRUCTOR.
Add doxygen group.
libpixi: install a pkgconfig file libpixi.pc
libpixi: add pixi-cc and pixi-c++ scripts
extensions: remove source dependency on pio
libpixi: improvements to library version handling

Make version.h a public header.
Use a macro instead of string constant.
libpixi: add checks for expected library version
extensions: add a pixi-ext application
pio: load-fpga: print FPGA build time when finished
libpixi: rename the pi gpio functions from pixi_gpio to pixi_piGpio

This is to allow pixi gpio functions to be renamed from pixi_pixiGpio to
pixi_gpio.
libpixi: add LIBPIXI_MUST_CHECK/LIBPIXI_USE_RESULT macros
libpixi: fix for set mode on pixi GPIO