Port version 3 firmware (#15)

* Port version 3 firmware

* Convert PS/2 scancode to USB HID info

* Simplified VGA driver

* One DMA transfer per frame

* Byte-aligned VGA buffer

* Monochrome configuration

* Constant luminance table

* Yield to probe keyboard

* Fix keyboard LEDs

* Clear screen line for monochrome

* Use local buffer for echo mode

* Fix SD/SU

* Probe keyboard when in config UI

* Optimize character rendering

* Terminus bold and normal fonts. Write config to NVM. Config for flow control and inverted serial. Fix CHA.

* Start-up message

* Re-use terminal keyboard driver for config UI

* Non-polarized C1, C2 and C3

* IBM PC charset. HPR, VPR, REP, CHT, CBT controls. Separate receive and transmit C1 modes.

* Fix line-wrapping semantics according DEC STD-070 with wraptest

* Fix color adjustments when rendering character

* Handle secondary DA and DECSTR

* Adjust F1-F4 codes

* Support 30 rows mode to use full VGA resolution

* Shift-Alt-Ctrl modifiers

* Configurable monochrome transformation

* Alternative cells

* OC4 to generate optimal SPI framing. Remove support for screen buffer padding.

* Adjust config UI copy

* Clear screen when alternative cells are not supported

* 4K UART receive buffer

* Hardware changes for optimal SPI framing

* Keyboard compatibility: PC/Sun and VT220

* Externalize tab stops and cells memory. Use bitset for tab stops.

* Keypad mode and keyboard led initialization

* Fix setting initial keyboard LEDs

* Process PS/2 keyboard command response. Detect keyboard.

* 3.0