[wf-pacman] Populate RootDir using relative paths

[asiekierka]

wf-pacman is always installed in $WONDERFUL_TOOLCHAIN/bin/wf-pacman, and RootDir is always $WONDERFUL_TOOLCHAIN. Rework wf-pacman's directory cofniguration to:

• assume RootDir is equal to $WONDERFUL_TOOLCHAIN if present (or $(dirname $0)/.. if not),
• configure all other paths relative to this RootDir.

This removes another hardcoded /opt/wonderful references.