Add an external watchdog module

[Quency-D]

1. It has already been tested on heltec-mesh-solar and heltec-v4.
2. The newly added watchdog module does not conflict with the existing meshsolar.

[0x156]

Found this from here: http://community.heltec.cn/t/heltec-meshtower-resets-its-configuration-itself/20570/36?u=brikots
I have flashed it to my Heltec mesh solar/tower, and I can confirm it works properly: it does not reboot every 9 minutes.

[weebl2000]

A few suggestions — the main one is a units mismatch (ms vs seconds) in the repeater sleep guard.

[Quency-D]

@weebl2000 Thank you for your suggestions. All have been revised.

[liamcottle]

Please refactor all instances of ex to external, so the language is more explicit.

E.g:

• HAS_EX_WATCHDOG -> HAS_EXTERNAL_WATCHDOG
• EX_WATCHDOG_DONE_PIN -> EXTERNAL_WATCHDOG_DONE_PIN
• EX_WATCHDOG_WAKE_PIN -> EXTERNAL_WATCHDOG_WAKE_PIN
• EX_WATCHDOG_TIMEOUT_MS -> EXTERNAL_WATCHDOG_TIMEOUT_MS
• ExWatchdogManager -> ExternalWatchdogManager
• SolarExWatchdog -> SolarExternalWatchdog
• ex_watchdog -> external_watchdog

[weebl2000]

@Quency-D addressed the review comments and opened a PR to your branch Quency-D#4

[weebl2000]

@liamcottle addressed the comments, variables are clearly named now!

[Quency-D]

@Quency-D addressed the review comments and opened a PR to your branch Quency-D#4

Thank you for your contribution; the connection has been merged.