testsuite/mk/boilerplate.mk: rename 'ghc-config-mk' to 'ghc_config_mk'

Make/shell variable names which contain dashes can cause problems under
some conditions. The 'ghc-config-mk' variable from testsuite/mk/boilerplate.mk
that I made overridable (by Hadrian) in ba0aed2e was working as expected when
our Hadrian/Linux job was based off the deb8 Docker image, but broke when
I switched the job to use our deb9-based image, in 3d97bad6. The exact
circumstances/tool versions that trigger this problem are unknown, but
changing the variable's name to 'ghc_config_mk' lets us work around the issue.

This fixes the annth_compunits and annth_make test failures that showed up
when we switched the Hadrian/Linux job to use the deb9 environment.