applet.audio_output: new applet.

cli: for build, --rev is required.

target.hardware: factor in the platform choose logic. This used to live in cli, which is definitely not the right place. This commit also fixes tests.

target.hardware: explicitly tristate unused pins. This was already fixed in 71bcdbbe, but on revC, the default pullup on iCE40 IOs and the truth table of the level shifters means that any undriven OE FPGA pins will cause the corresponding level shifter to drive 1.

software: adapt for revC.

firmware: adapt for revC.

applet.selftest: update after 71bcdbbe7.

firmware: fix more illegal IOx manipulation. See 19662930 for details.

platform: remove IOStandard directives. NFC. They don't do anything on iCE40.

platform: split ports into individual bits. This is a more elegant solution for #91.