Fixes #1183 — T-Echo Lite incorrect battery voltage

[pelgraine]

The T-Echo Lite variant files were copied from the T-Echo without adapting to the Lite's different hardware. Three issues fixed, plus build environments added for the screenless Core variant.

Battery (TechoBoard.h/cpp): Added PIN_VBAT_MEAS_EN (P0.31) — the T-Echo Lite has a gated voltage divider that must be enabled before reading. Also added pinMode(PIN_VBAT_READ, INPUT) before each ADC read to reclaim P0.02 from other peripherals. Pin definitions hardcoded from LilyGo's t_echo_lite_config.h.

I2C (variant.h): Corrected SDA/SCL from P0.04/P0.02 to P1.04/P1.02 per LilyGo's IIC_1_SDA/IIC_1_SCL. The old P0.02 mapping conflicted with the battery ADC pin.

GPS (variant.h): Corrected all five GPS pin assignments to match LilyGo's config — UART TX/RX, wake, PPS, and power enable were all scrambled.

Tested on T-Echo Lite Base (screenless):

Battery readings now match Heltec V4 comparison reference within 10mV - Heltec v4 read partially discharged battery as 3.51v, screenshots of T-Echo Lite readings below:

[jirogit]

Hi @pelgraine — a few questions to help clarify which variant this PR
actually targets, since LilyGO's naming is confusing many people
(including me):

1. Which hardware do you have? The PR title says "Core" but the
   photos in here looks Non-Shell. Could you confirm
   whether your device is:

   • T-Echo Lite Core (castellated module, no USB, no display)
   • T-Echo Lite Non-Shell (has USB-C, no display)
   • T-Echo Lite (full version with ePaper display)

2. Naming convention: The new base env LilyGo_T-Echo-Lite-Core
   uses a hyphen before "Core", while the rest of the codebase uses
   underscores (e.g. Heltec_t114_without_display_companion_radio_ble).
   Also, _base suffix is used for PlatformIO base envs elsewhere
   (e.g. Heltec_E213_base), while core does not appear anywhere
   in the codebase. Worth aligning with maintainer preference.

I ask because my PR #2353 targets the Non-Shell variant specifically,
and I want to make sure we cover all three variants correctly without
overlap or gaps.

[pelgraine]

@jirogit

Hi, yes, I shouldn't have put the platformio changes in. This fix is really just about the battery voltage problem detailed in issue #1183, not the headless build problem. The one I worked with was actually the T-Echo Lite Non-Shell (has USB-C, no display) version. When I have a spare minute I'll fix the PR to remove those.

[pelgraine]

Updated to remove platformio changes and references to headless env.

Also tested with my new T-Echo Lite with shell & screen with the T-Echo Lite companion radio BLE build and can confirm that displays as expected.

[pelgraine]

@jirogit all updated!