playbook/olimex-sensor: Drop old images support...

...for the purpose of a less obscure and more clear code.

The "dropped Olimex Debian images" are supposed to be very old.
Please see nexedi/slapos.package@7c5a2eff (comment 167627)
for a complete discussion of this decision.

playbook/roles/olimex-sensor/tasks/main.yml

@@ -17,20 +17,6 @@
     name: bme280
     executable: pip3
 
-# We need to set a different bus number depending
-# on the debian version. For Debian 12 we may
-# need another number again.
-
-- name: Set bus number for debian bullseye
-  set_fact:
-    i2c_bus_number: 3
-  when: ansible_distribution == 'Debian' and ansible_distribution_release == 'Buster'
-
-- name: Set bus number for debian bullseye
-  set_fact:
-    i2c_bus_number: 2
-  when: ansible_distribution == 'Debian' and ansible_distribution_release == 'bullseye'
-
 - name: Solve python template to read sensor
   template:
     src: read_bme280.j2

playbook/roles/olimex-sensor/vars/main.yml

-i2c_bus_number: "3"
+# This i2c bus number should relate to the port of the Olimex
+# board "A20-OLinuXino-LIME2", where the sensor is plugged in the
+# following picture:
+#
+#   https://wendelin.nexedi.com/wendelin-Tutorial.Setup.Sensor.IOTGateway/olimex-sensor.Photo?format=
+#
+# This picture belongs to the Wendelin Learning Track:
+#
+#   https://wendelin.nexedi.com/wendelin-Tutorial.Setup.Sensor.IOTGateway/WebPage_viewAsWeb
+#
+# This i2c bus number is expected to represent the given port
+# if the running OS is one of the Olimex provided Debian images
+# (starting from Debian 11). It is possible that at any given
+# moment Olimex may decide to change the i2c bus number mapping.
+# The mapping is defined in the following devicetree:
+#
+#   https://github.com/torvalds/linux/blob/4e0d439/arch/arm/boot/dts/sun7i-a20.dtsi#L1439-L1450
+#
+# If respective parts in this file are changed, we may need to
+# adapt the 'i2c_bus_number' if the playbook shall work with
+# newer Olimex Debian images.
+#
+# Please find more information in the following note:
+#
+#   https://lab.nexedi.com/nexedi/slapos.package/commit/7c5a2effb5d7f25d3753f31db3bb7fcf44f2ebd2#note_167627
+i2c_bus_number: "2"