Reduce firmware size on ESP8266

[brentru]

This pull request reduces the ESP8266 build's size by -34.1%.

Changes:

• On ESP8266, we now use the F() macro to store string literals in Flash (PROGMEM) instead of RAM. This saves precious RAM on memory-constrained ESP8266 devices.
  • NOTE: WS_DEBUG_PRINT/PRINTLN only accepts string literals on ESP8266, so use WS_DEBUG_PRINTVAR/PRINTLNVAR for variables on this platform.
  • NOTE: For other platforms, use the standard variadic macros
• PROGMEM macro introduced in display/ for storing icons and splash screen assets

Build Size Comparison

main branch:

RAM:   [==========]  96.9% (used 79396 bytes from 81920 bytes)
Flash: [======    ]  64.7% (used 675507 bytes from 1044464 bytes)

This branch:

RAM:   [======    ]  62.8% (used 51456 bytes from 81920 bytes)
Flash: [=======   ]  65.0% (used 678923 bytes from 1044464 bytes)

Addresses:
#841 (needs fwd port to v2 after this PR is in)
#862

[brentru]

@tyeth in addition to reviewing, could you test on an ESP32 or ESP32-x platform to ensure the F() macro changes didn't introduce any unusual behavior on the Serial?

[brentru]

works on 8266

[DoctorWho8]

So this now works?