Fix gzip+signed OTA error (esp8266#7577)

The last 4 bytes of a GZIP file is the decompressed file length, and
are used in eboot to do sanity checks and know when decompression is
done.

Updater was incorrectly telling eboot to look at
"end-of-bin + sizeof(signing)", and when eboot did so it got an
incorrect value causing either the update to be skipped or for only a
portion of update to be completed.

Fix by adjusting the size back to the end of binary.
Fixes esp8266#7570

cores/esp8266/Updater.cpp

@@ -282,6 +282,8 @@ bool UpdaterClass::end(bool evenIfRemaining){
       return false;
     }
     free(sig);
+    _size = binSize; // Adjust size to remove signature, not part of bin payload
+
 #ifdef DEBUG_UPDATER
     DEBUG_UPDATER.printf_P(PSTR("[Updater] Signature matches\n"));
 #endif