Fix GITHUB action build.

PR #119 made that no machine-type is selected, so the build will fail.
commit 2f787fee enables us select a machine via a variable, use this to
select a machine-type `KP_9000_6XHML_X2` to build.

.github/workflows/build.yml

@@ -18,4 +18,4 @@ jobs:
           apt update
           apt install make gcc sdcc xxd python-is-python3 libjson-c-dev -y
       - name: Make project
-        run: make
+        run: make MACHINE="KP_9000_6XHML_X2"