Toolchain makefile variables break Verissimo integration

Created by: gabrielrad

When invoking make create_dvt_build_file from core-v-verif/cv32e40p/sim/uvmt directory, to create the build file needed by Verissimo in order to compile the project we get:

core-v-verif/mk/Common.mk:83: Must define a DPI_DASM_SPIKE_REPO to use the common makefile core-v-verif/mk/Common.mk:253: *** CV_SW_TOOLCHAIN not defined in either the shell environment, test.yaml or cfg.yaml. Stop.