[BOOTVID] Diverse enhancements.

[BOOTVID] Diverse enhancements.

- Import cmdcnst.h and vga.h headers from the 'vga_new' VGA Miniport Driver,
  that contain definitions related to VGA registers as well as command-stream
  functionality.

- Replace a bunch of hardcoded values by their corresponding defintions.

- Replace "Captain-Obvious" comments in VgaIsPresent() with actual
  explanations from the corresponding function in 'vga_new'.

- Simplify the VgaInterpretCmdStream() function, based on the
  corresponding one from 'vga_new'.

- Use concise comments in the 'AT_Initialization' command stream definition.

- Import the 'VGA_640x480' initialization command stream from 'vga_new'
  and use it as the full VGA initialization stream whenever the HAL
  does not handle the VGA display (HalResetDisplay() returning FALSE).
  Otherwise we just use the 'AT_Initialization' command stream that
  performs minimal initialization.

- Remove unused AT_Initialization and other declarations from ARM build.