Issue: Sometimes when an ESP8266 is reflashed/upgraded the WiFi does not work.
Then a  serial flash with Erase Flash with WiFi setting is recommended. I have
seen this more often when changing SDK by OTA. We don't have an erase WiFi for
OTA.

The PR tries to present a proof of concept solution. Actually it describes two
different methods.

There are 3 cases to consider when the firmware is updated by OTA. The new
firmware:
1. has the same Flash Configuration as the old.
2. has a larger Flash Configuration than the old.
3. has a smaller Flash Configuration then the old.

In theory after an OTA and before a restart, the flash could be erased for
_case 1_. _Case 2_ is a problem because the size exceeds what the SPIEraseSector
expects and fails. We have to wait for a restart, after which the SDK will have
updated the values in `flashchip`. _Case 3_ is potentially unsafe because we
could be erasing the code that is running.

At app_entry() `flashchip` properties appear to be reset to a default 4MByte
flash. Even an ESP8285 reported 4MByte. The value of `flashchip->chip_size` was
changed at the 2nd SPIRead. The 1st read was to address 0, 4 bytes.

To erase the flash WiFi area, I choose to wait until the SDK has finished its
adjustments to the `flashchip` structure. Then to begin erasing WiFi sectors.

I implemented two methods:
1. The first runs after the SDK calls `user_init()` and flash code execution is
available (No IRAM needed), but restarts to be sure the SDK  does not get
confused about its sectors being erased.
2. The 2nd method runs as early as possible required IRAM. The sectors are
erased before that 2nd read is processed. The SDK allowing the SDK to start off
thinking the sectors were blank at boot.

I also added an example/test sketch for exercising the feature. It is
OTAEraseConfig.

…tion.

And code to ensure the UART speed stayed at 74880 for a more complete
viewing of messages. Well at least, it stays at 74880 for a longer time than it was.
Added pinMode filter to prevent UART TX pin from being disabled during user_init().

This is just a PoC an alternate for preinit() can be created later.

This method runs before the NONOS SDK is started. Patches the flashchip
structure that the ROM APIs reference for validating input before calling
SPIEraseSector.

Extracted logic from EspClass::magicFlashChipSize() to make a "C"
callable function and made EspClass::magicFlashChipSize() a thin wrapper
to avoid duplicating code.

Use Cache_Read_Enable() to enable flash code execution to reduce IRAM
usage by pre SDK starup code.

Improved comments and made comment formating more consistant in erase_config.cpp.

Improved issolation of debug printing.

Changed method for reading image header, freeing 16 bytes of iRAM"

Latest merge broke my TZ logic. Updated to the new configTime method.

Updated WifiHealth to handle multiple/duplicate disconnects w/o connects.
When a AP goes off line, a disconnect callback is done for each failed
connect attempt. Before the total connect time was getting inflated.

Now uses LED_BUILTIN for WiFi Up/Down state.

Updated WifiHealth.ino to report total time connected and disconnected.
"Connection Uptime" is the time since last connect.
"Total Uptime" is the sum of all past uptimes.
"Total Downtime" is the sum of all past downtimes.
"min/max/avg" reportings are of past uptimes or past downtimes.

Adjust comments for style?

…ure.

Using static caused the compiler to inline with functions with different IRAM
attributes. Updated some comments. Maybe fixed CI host issue with Update.cpp.

Method 3 was failing with an unaligned access due to 32-bit word access
being optomized to an 8-bit when accessing ICACHE memory.
Added noinline with IRAM_CACHE attribute in module erase_config.cpp.
Also, revised print macros to be more globally unique.

advange of C++ element initialization options.

method of handling direct ROM calls.

…anges.

Fixed missed edit (rom_uart_div_modify) for debug build.

…sions

in esp8266#8261 (comment)

…o pr-erase-config