Ported from meshcore-dev/MeshCore#2140 (Quency-D <hj_zzns@163.com>).
The upstream v1.16.0 merge changed the Heltec FEM LNA to disabled by
default (upstream commit 696aae6e). This patch restores the enabled
default for repeater via the new "radio.fem.rxgain" preference
(defaulted to 1), while also making the setting user-configurable
and persisted across reboots.
The GC1109 FEM needs its VFEM_Ctrl pin held HIGH during deep sleep
to keep the LNA active, enabling proper RX sensitivity for
wake-on-packet. Without this, the LNA is unpowered during sleep
and RX wake sensitivity is degraded by ~17dB.
Release RTC holds in begin() after configuring GPIO registers
(not before) to ensure glitch-free pin transitions on wake.
Trade-off: ~6.5mA additional sleep current for significantly
improved wake-on-packet range.
Valentin V. Bartenev