Micropython: More reliable esp8266 binary downloads

[pfalcon]

Some points were brought up elsewhere:

1. If a build fails, there's no binary available for download (should keep previous around).
2. In general, it would be nice to keep 3-4 last builds, to let users and maintainers to catch/compare behavior in case of recently introduced/non-deterministic bug.

[deshipu]

If you are editing that page, it would also be great to add a link to flashing instructions right there.

[dpgeorge]

I have implemented the following changes to the download page:

• last 4 daily builds are kept for esp8266 port
• removed hyphens from date format in all firmware filenames to make filenames easier to read
• added shortcut links at the top of the page to the firmware sections
• added link in the esp8266 port section to flashing instructions in the tutorial

In a few days we will see if the changes are all working.

[pfalcon]

This was implemented (may need tweaks/adjustments to handle esp8266 releases).