Upper palette chars support reverse, blink, underline; rewrote build instructions

This change to VIC-III character attributes in the regular color mode causes reverse, blink, and underline to behave identically for both lower palette (non-"bold") and upper/alternate palette ("bold") characters.

Previously, the alt palette bit was being treated as a blinkable attribute, similar to how blink+reverse actually blinks between reverse and non-reverse. Blinking between the two palettes is not useful or intuitive behavior, and it rules out more useful possibilities like blinking an alt palette character on and off. With this change, bit 6 always sets the alt palette with no interaction with blink, and the other attributes behave as expected.

Note that this redefines the glyph_bold_drive line to not blink either. This is now defined as "bit 6 (bold) but not bit 5 (reverse)," which is a requirement elsewhere in the logic to enable reverse alt-palette characters.

Implements this request: https://github.com/MEGA65/mega65-core/issues/715
Fixes this cursor color bug in the latest ROM: https://github.com/MEGA65/mega65-rom-public/issues/56

Unrelated but included: I have rewritten the core build documentation based on my recent attempt to set up core builds from scratch on a new Linux system.

MEGA65 / mega65-core

Upper palette chars support reverse, blink, underline; rewrote build instructions #716