conditional: Document the bootvar0 variable

The `bootvar0` variable is a 32-bit user defined value which is set through
rpi-eeprom-config, and then can be used as a conditional variable in config.txt

Author: popcornmix

documentation/asciidoc/computers/config_txt/conditional.adoc

@@ -171,6 +171,26 @@ sudo vcmailbox 0x0003008c 8 8 1 0
 ----
 The value of the `boot_arg1` variable when the OS was started can be read via xref:configuration.adoc#part4[device-tree] at `/proc/device-tree/chosen/bootloader/arg1`
 
+==== `bootvar0`
+Raspberry Pi 5 and newer devices only.
+
+The `bootvar0` variable is a 32-bit user defined value which is set through rpi-eeprom-config, and then can be used as a conditional variable in config.txt.
+
+For example, setting `bootvar0` to 42 via `rpi-eeprom-config`:
+[source,ini]
+----
+BOOTVAR0=42
+----
+
+and then using it conditionally in `config.txt`:
+[source,ini]
+----
+[bootvar0=42]
+arm_freq=1000
+----
+
+This allows a common image (i.e. with the same config.txt file) to support different configurations based on the persistent rpi-eeprom-config settings.
+
 ==== `boot_count`
 Raspberry Pi 5 and newer devices only.