Add command to extract build info from metadata

This will allow to recreate the metadata after patching the FS.

PUBLISHED_FROM=dae228bbf49d71f383e4dc52316cf98aba3914f5

docs/design-concept/memory-buffers.md

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+---
+title: Memory buffers
+---
+
+Each connection has send and receive buffer,
+[`struct mg_connection::send_mbuf`](#TODO)
+and
+[`struct mg_connection::recv_mbuf`](#ODO) respectively.
+When data arrives,
+Mongoose appends received data to the `recv_mbuf` and
+triggers `MG_EV_RECV` event. User may send data back by calling one of the
+output functions, like [`mg_send()`](#TODO) or
+[`mg_printf()`](#TODO). Output functions append data to the
+`send_mbuf`. When Mongoose
+successfully writes data to the socket, it discards data from
+[`mg_connection::send_mbuf`](#TODO) and
+sends `MG_EV_SEND` event. When connection is closed, `MG_EV_CLOSE` event is sent.
+
+![](../../static/img/mongoose/mbuf.png)
+