obs/generic/: Add notes on the differences between the slapos-node packaging...

obs/generic/: Add notes on the differences between the slapos-node packaging system and the generic packaging system.

obs/generic/diff_slapos-node_generic.txt

+# Variable name changes between the slapos-node packaging system and the generic packaging system.
+
+# BUILD_DIR was formerly BUILD_ROOT_DIRECTORY
+BUILD_DIR="$TARBALL_DIR/build/"
+
+# RUN_BUILDOUT_DIR is where the parts/ directory will be ; formerly BUILD_DIRECTORY
+RUN_BUILDOUT_DIR="$BUILD_DIR/$TARGET_DIR"