add lateInitVariant() as a concept. see below for docs

(from src/extra_variants/README.md)

This directory tree is designed to solve two problems.

- The ESP32 arduino/platformio project doesn't support the nice "if initVariant() is found, call that after init" behavior of the nrf52 builds (they use initVariant() internally).
- Over the years a lot of 'board specific' init code has been added to init() in main.cpp. It would be great to have a general/clean mechanism to allow developers to specify board specific/unique code in a clean fashion without mucking in main.

So we are borrowing the initVariant() ideas here (by using weak gcc references). You can now define lateInitVariant() if your board needs it.

If you'd like a board specific variant to be run, add the variant.cpp file to an appropriately named
subdirectory and check for \_VARIANT_boardname in the cpp file (so that your code is only built for your board).
You'll need to define \_VARIANT_boardname in your corresponding variant.h file.
See existing boards for examples.

This approach has no added runtime cost.

variants/heltec_wireless_tracker/variant.h

@@ -1,5 +1,6 @@
 #define LED_PIN 18
 
+#define _VARIANT_HELTEC_WIRELESS_TRACKER
 #define HELTEC_TRACKER_V1_X
 
 // I2C
@@ -31,7 +32,8 @@
 // GPS UC6580:          GPS V_DET(8), VDD_IO(7), DCDC_IN(21), pulls up RESETN(17), D_SEL(33) and BOOT_MODE(34) through 10kR
 // GPS LNA SW7125DE:    VCC(4), pulls up SHDN(5) through 10kR
 // LED:                 VDD, LEDA (through diode)
-#define VEXT_ENABLE 3 // active HIGH - powers the GPS, GPS LNA and OLED VDD/anode
+
+#define VEXT_ENABLE 3 // active HIGH - powers the GPS, GPS LNA and OLED
 #define VEXT_ON_VALUE HIGH
 #define BUTTON_PIN 0