Add documentation for compressed OTA, fix bug

Fixes #6923

Documents the user steps needed to do a compressed upload, and notes the
2-step process needed for deploying compressed uploads to the field for
the first time.

Fixes a bug in serial output formatting discovered by @AdrianEddy.

Adds additional contributors for uzlib, per @pfalcon.

README.md

@@ -124,7 +124,11 @@ ESP8266 core files are licensed under LGPL.
 
 [LittleFS](https://github.com/ARMmbed/littlefs) library written by ARM Limited and released under the [BSD 3-clause license](https://github.com/ARMmbed/littlefs/blob/master/LICENSE.md).
 
-[uzlib](https://github.com/pfalcon/uzlib) library written and (c) 2014-2018 Paul Sokolovsky, licensed under the ZLib license (https://www.zlib.net/zlib_license.html).
+[uzlib](https://github.com/pfalcon/uzlib) library written and (c) 2014-2018 Paul Sokolovsky, licensed under the ZLib license (https://www.zlib.net/zlib_license.html).  uzlib is based on:
+* tinf library by Joergen Ibsen (Deflate decompression)
+* Deflate Static Huffman tree routines by Simon Tatham
+* LZ77 compressor by Paul Sokolovsky
+* Library integrated and maintained by Paul Sokolovsky.
 
 ### Other useful links ###
 

bootloaders/eboot/eboot.c

@@ -27,14 +27,14 @@ int print_version(const uint32_t flash_addr)
     if (SPIRead(flash_addr + APP_START_OFFSET + sizeof(image_header_t) + sizeof(section_header_t), &ver, sizeof(ver))) {
         return 1;
     }
-    char fmt[16];
+    char fmt[7];
     fmt[0] = 'v';
     fmt[1] = '%';
     fmt[2] = '0';
     fmt[3] = '8';
     fmt[4] = 'x';
     fmt[5] = '\n';
-    fmt[6] = '0';
+    fmt[6] = 0;
     ets_printf((const char*) fmt, ver);
     return 0;
 }

doc/ota_updates/readme.rst

@@ -123,7 +123,7 @@ Compile the sketch normally and, once a `.bin` file is available, sign it using
 
 .. code:: bash
 
-    <ESP8266ArduioPath>/tools/signing.py --mode sign --privatekey <path-to-private.key> --bin <path-to-unsigned-bin> --out <path-to-signed-binary>
+    <ESP8266ArduinoPath>/tools/signing.py --mode sign --privatekey <path-to-private.key> --bin <path-to-unsigned-bin> --out <path-to-signed-binary>
 
 Old And New Signature Formats
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -134,7 +134,34 @@ To create a legacy signature, call the signing script with --legacy:
 
 .. code:: bash
 
-    <ESP8266ArduioPath>/tools/signing.py --mode sign --privatekey <path-to-private.key> --bin <path-to-unsigned-bin> --out <path-to-signed-binary> --legacy <path-to-legacy-file>
+    <ESP8266ArduinoPath>/tools/signing.py --mode sign --privatekey <path-to-private.key> --bin <path-to-unsigned-bin> --out <path-to-signed-binary> --legacy <path-to-legacy-file>
+
+
+Compression
+-----------
+
+The eboot bootloader incorporates a GZIP decompressor, built for very low code requirements.  For applications, this optional decompression is completely transparent.  For uploading compressed filesystems, the application must be built with `ATOMIC_FS_UPDATE` defined because, otherwise, eboot will not be involved in writing the filesystem.
+
+No changes to the application are required.  The `Updater` class and `eboot` bootloader (which performs actual application overwriting on update) automatically search for the `gzip` header in the uploaded binary, and if found, handle it.
+
+Compress an application `.bin` file or filesystem package using any `gzip` available, at any desired compression level (`gzip -9` is recommended because it provides the maximum compression and uncompresses as fast as any other compressino level).  For example:
+
+.. code:: bash
+
+    gzip -9 sketch.bin  # Maximum compression, output sketch.bin.gz
+    <Upload the resultant sketch.bin.gz>
+
+If signing is desired, sign the gzip compressed file *after* compression.
+
+.. code:: bash
+
+    gzip -9 sketch.bin
+    <ESP8266ArduinoPath>/tools/signing.py --mode sign --privatekey <path-to-private.key> --bin sketch.bin.gz --out sketch.bin.gz.signed
+
+Updating apps in the field to support compression
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you have applications deployed in the field and wish to update them to support compressed OTA uploads, you will need to first recompile the application, then _upload the uncompressed `.bin` file once_.  Attempting to upload a `gzip` compressed binary to a legacy app will result in the Updater rejecting the upload as it does not understand the `gzip` format.  After this initial upload, which will include the new bootloader and `Updater` class with compression support, compressed updates can then be used.
 
 
 Safety