VLEN should not be set by preprocessor macro in ara_pkg.sv

[jerryz123]

This makes integration in other projects cumbersome. VLEN should follow same pattern as NR_LANES... ara_tb should set the VLEN parameter from the macro, but ara_pkg should not reference the macro.

[mp-17]

Hello @jerryz123, you are right. We have done it in the beginning to speed up the development but now it's really ugly.
We are moving to a more realistic SoC now, and I have already made VLEN a verilog parameter to solve this.

[mp-17]

Solving in #317