samples: minimal: define an empty project configuration

Cmake configuration forces projects to define a prj.conf

Signed-off-by: Romain Mahoux <romain@mahoux.fr>
diff --git a/samples/basic/minimal/prj.conf b/samples/basic/minimal/prj.conf
new file mode 100644
index 0000000..565c7da
--- /dev/null
+++ b/samples/basic/minimal/prj.conf
@@ -0,0 +1 @@
+# empty config