Add ATmega build and flash script

Use Dockerfile for firmware compilation and avrdude for flashing
to ATmega328p via usbasp.

src/atmega/build-atmega.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -e
+
+IMAGE_NAME="plant-manager-builder"
+SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+IMAGE_NAME="plant-manager-builder"
+DOCKERFILE="$SCRIPT_DIR/Dockerfile"
+BUILD_CTX="$SCRIPT_DIR"
+DEST_HEX="$SCRIPT_DIR/build/moisture.hex"
+MCU="atmega328p"; PRG="usbasp"; AVR_FLAGS="-B 10"
+
+docker build -t "$IMAGE_NAME" -f "$DOCKERFILE" "$BUILD_CTX"
+
+mkdir -p "$(dirname "$DEST_HEX")"
+
+docker run --rm "$IMAGE_NAME" \
+  cat /usr/src/build/moisture.hex > "$DEST_HEX"
+
+command -v avrdude >/dev/null || { echo "Install avrdude"; exit 1; }
+avrdude -v -p "$MCU" -c "$PRG" $AVR_FLAGS -D -U flash:w:"$DEST_HEX":i