ICE40 Feather support

[whatnick]

• Support pinmaps for LED
• Support USB-UART
• Support SPI/Quad-SPI
• Support using "icefeather" as a platform string
• Verify with physical board the lm32 and vexriscv micropython startup
• Support icefeather profile build in travis CI matrix
• Support Feather pins for i2c, spi and second UART
  Signed-off-by: Tisham Dhar whatnickd@gmail.com

[todo]

Do not need these

litex-buildenv/platforms/ice40_feather.py

Lines 24 to 29 in bead23a

	# TODO: Do not need these
	Subsignal("wp", Pins("12"), IOStandard("LVCMOS33")),
	Subsignal("hold", Pins("13"), IOStandard("LVCMOS33")),
	),
	("spiflash4x", 0,

---

This comment was generated by todo based on a TODO comment in bead23a in #614. cc @whatnick.

[todo]

Find out quad-spi pins

litex-buildenv/platforms/ice40_feather.py

Lines 32 to 37 in bead23a

	# TODO: Find out quad-spi pins
	Subsignal("dq", Pins("14 17 12 13"), IOStandard("LVCMOS33")),
	),
	("clk12", 0, Pins("35"), IOStandard("LVCMOS33"))
	]

---

This comment was generated by todo based on a TODO comment in bead23a in #614. cc @whatnick.

[todo]

Inferred tristate not currently supported by nextpnr; upgrade

litex-buildenv/targets/icefeather/base.py

Lines 77 to 82 in c802ff8

	# TODO: Inferred tristate not currently supported by nextpnr; upgrade
	# to spiflash4x when possible.
	self.submodules.spiflash = spi_flash.SpiFlashSingle(
	platform.request("spiflash"),
	dummy=platform.spiflash_read_dummy_bits,
	div=platform.spiflash_clock_div,

---

This comment was generated by todo based on a TODO comment in c802ff8 in #614. cc @whatnick.

[whatnick]

This is the WIP for ice-feather support. Progress so far is

• IceFeather can be made to work with soft-cpu + micropython by hacking ice-breaker definition with some pin changes
• IceFeather is recognized as a platform using an environment variable. However the gateware + firmware images built do not boot into a working micropython.

Making this up as I go along, would benefit from a adding new board HOW-TO. The boards is very very similar to icebreaker.