Streamline installation process (Spike and toolchain variables, README file).

Created by: zchamski

List of changes:

[Spike] Phase out SPIKE_ROOT in Spike installation script.

• verif/regress/install-spike.sh: Replace occurrences of SPIKE_ROOT with SPIKE_INSTALL_DIR. Add coments.

Streamline installation process.

• README.md (Prerequisites): Update.
• verif/regress/install-cva6.sh (CV_SW_PREFIX): Add comment on value auto-detection.
  (RISCV_GCC): Ditto.
  (RISCV_OBJCOPY): Ditto.
  (SPIKE_ROOT): Replace with SPIKE_INSTALL_DIR.
  
• verif/regress/install-riscv-dv.sh (SPIKE_ROOT): Ditto.
• verif/regress/install-spike.sh (ROOT_PROJECT): Set automatically from script path at all times.
  (SPIKE_ROOT): Phase out.
  (build+install procedure): Update log messages.
  
• verif/sim/cva6.py (main): Replace SPIKE_ROOT with SPIKE_INSTALL_DIR.

Phase out SPIKE_ROOT as installation location variable.

• verif/regress/install-spike.sh: Replace occurrences of SPIKE_ROOT with SPIKE_INSTALL_DIR. Add coments.