ThinkNode M6: make GPS usable in the repeater and room server builds

[andyhoobing]

Split out of #3287 as one self-contained fix, per the one-fix-per-PR guideline.
The companion PR covers the M6 status LED and shares no files with this one.

Problem

On a sealed Elecrow ThinkNode M6 (nRF52840 + SX1262 + soldered-on L76K GNSS,
IP65 solar enclosure) the GNSS could not be used at all:

1. simple_repeater and simple_room_server both default gps_enabled = 0, so
   sensors.begin() detected the L76K and then immediately powered it back down
   via _location->stop() (which drives PIN_GPS_EN low). The M6 has no button
   or screen, so there was no way to switch it on short of a USB console.
2. Both default advert_loc_policy to ADVERT_LOC_PREFS, so adverts carried
   the fixed ADVERT_LAT/ADVERT_LON (0,0) and ignored the GNSS entirely.
3. The 1-second "is a GPS attached?" probe in initBasicGPS() gates the gps
   setting; losing that race leaves GPS unreachable until reboot. The M6 always
   has the module fitted.

Approach

Rather than changing behaviour for every board, the two example defaults become
build-time overridable:

#ifndef GPS_ENABLED_DEFAULT
  #define GPS_ENABLED_DEFAULT 0
#endif
#ifndef ADVERT_LOC_POLICY_DEFAULT
  #define ADVERT_LOC_POLICY_DEFAULT ADVERT_LOC_PREFS
#endif

Every other board keeps its existing defaults. Only the M6 repeater and
room_server envs opt in, and they also set ENV_SKIP_GPS_DETECT. Both
settings remain adjustable at runtime via gps on/gps off and
gps advert prefs|share|none.

Each example declares the defines where it already keeps its ADVERT_*
fallbacks — MyMesh.cpp for the repeater, MyMesh.h for the room server — to
match each example's own convention rather than imposing one layout on both.

Testing

• ThinkNode_M6_repeater, ThinkNode_M6_room_server, RAK_4631_repeater and
  RAK_4631_room_server all build against dev.
• pio run -t envdump confirms the new flags resolve for the M6 envs and are
  absent for the RAK envs, so other boards are provably untouched.
• Confirmed on real M6 hardware over the serial CLI: gps -> on, active, fix, 13 sats, gps advert -> share, surviving a reboot.

Note for maintainers

One pre-existing behaviour worth being aware of (not changed by this PR): these
compiled defaults only apply to a fresh install. A UF2 update does not erase
InternalFS, so a device that has run MeshCore before keeps its saved prefs and
silently ignores new defaults — it must be reconfigured at runtime or factory
reset. This cost me some debugging and may be worth documenting.

---

Authored with assistance from Claude (see Co-Authored-By trailers).