doc: added descriptions and images for current nodes n29-n30.

doc/mcu-examples/n28/0BUILD

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+<?xml?>
+
+<gwbuild>
+
+  <extradist>
+    README.md
+    n28_pcb_back.png
+    n28_pcb_front.png
+    n28_schematic.png
+  </extradist>
+
+</gwbuild>
+

doc/mcu-examples/n28/README.md

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+
+N28
+===
+
+
+N28 is a ATtiny84 based PCB with a door sensor and a temperature/humidity
+sensor.
+
+
+
+Overview
+--------
+
+- Role: Door sensor with temp sensor
+- MCU: AtTiny84
+- Connection: RJ45
+- COM: COM2W
+- Predecessor: N21, N16
+- Periphery:
+  - door sensor (TCRT1000)
+  - TWI interface
+    - SI7021 temperature and humidity sensor
+- Apps:
+  - NETWORK      : Basic network functionality (address setup etc.)
+  - REPORTSENSORS: report sensor data from temperature sensor etc.
+  - STATS        : periodically send stats data
+  - DOOR         : watch door/window sensor and notify about state changes
+
+- Values:
+  - SI7021_TEMP  : temperature
+  - SI7021_HUM   : humidity
+  - DOOR         : state of door/window (0=closed, >0=open)
+
+