lib/littlefs: Add README describing origin and how to gen lfs1/lfs2.

Author: dpgeorge

lib/littlefs/README.md

@@ -0,0 +1,19 @@
+littlefs library
+================
+
+The upstream source for the files in this directory is
+https://github.com/ARMmbed/littlefs
+
+To generate the separate files with lfs1 and lfs2 prefixes run the following
+commands in the top-level directory of the littlefs repository (replace the
+version tags with the latest/desired ones, and set `$MPY_DIR`):
+
+    git checkout v1.7.2
+    python2 ./scripts/prefix.py lfs1
+    cp lfs1*.[ch] $MPY_DIR/lib/littlefs
+    git reset --hard HEAD
+
+    git checkout v2.1.3
+    python2 ./scripts/prefix.py lfs2
+    cp lfs2*.[ch] $MPY_DIR/lib/littlefs
+    git reset --hard HEAD