Refactor VIDEO_ATTR variable assignments

[binarymaster]

VIDEO_ATTR variable is used to change text color attributes, which is printed via printk() function.

It has 32-bit unsigned value which is represented in ARGB color space, 8 bits per channel:

VIDEO_ATTR = 0xAARRGGBB; // pseudocode

VIDEO_ATTR = 0xFFFF0000; // red color
VIDEO_ATTR = 0xFF00FF00; // green color
VIDEO_ATTR = 0xFF0000FF; // blue color
etc.

Note that the alpha (transparency) channel seems to be not implemented, and its value is always equal to 0xFF in the existing code. Changing the value of this attribute does not affect printed text visually (at least in XQEMU).

All assignments to this variable should be converted into C macros for better code self-documentation.