Now the underlying basevm support passes these along we can expose
some additional variables to our Makefile to allow more customised
tweaking of the build. For example:

  make vm-build-freebsd TARGET_LIST=aarch64-softmmu \
    EXTRA_CONFIGURE_OPTS="--disable-tools --disable-docs" \
    BUILD_TARGET=check-softfloat
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>