PARTITION parameters: Preliminary documentatiobn for remapping partition numbers

The latest firmware provides a basic method for remapping a high
partition number to a regular partition. This makes it more flexible
as generic reboot parameter.
E.g. It would be possible reboot into a different boot-order or
even network install.
This will probably evolve as we use it with rpi-image-gen

Author: timg236

documentation/asciidoc/computers/config_txt/conditional.adoc

@@ -96,6 +96,16 @@ Some models of Raspberry Pi, including Zero, Compute Module, and Keyboard models
 
 The `[none]` filter prevents any settings that follow from being applied to any hardware. Although there is nothing that you can't do without `[none]`, it can be a useful way to keep groups of unused settings in config.txt without having to comment out every line.
 
+=== The `[partition=N]` filter
+This `partition` filter can be be used in the bootloader config (only) to remap high partition numbers set either by the `sudo reboot N` command or direct usage of the hardware watchdog registers.
+
+Example
+[source,ini]
+---
+[partition=62]
+PARTITION=2
+---
+
 === The `[tryboot]` filter
 
 This filter succeeds if the `tryboot` reboot flag was set.

documentation/asciidoc/computers/raspberry-pi/eeprom-bootloader.adoc

@@ -499,6 +499,16 @@ Max length: 32 characters
 The `PARTITION` option may be used to specify the boot partition number, if it has not explicitly been set by the `reboot` command (e.g. `sudo reboot N`) or by `boot_partition=N` in `autoboot.txt`.
 This could be used to boot from a rescue partition if the user presses a button.
 
+The latest firmware also allows high partition numbers (> 31) to be overriden. This allows a custom setup of the system hardware watchdog to be trigger a reboot with a special high partition number (e.g. 62) which can be detected by the bootloader (using a conditional filter) and remapped to a recovery partition.
+
+Example:
+[source,ini]
+---
+# System watchdog fired - boot the rescue partition
+[partition=62]
+PARTITION=2
+---
+
 [source,ini]
 ----
 # Boot from partition 2 if GPIO 7 is pulled low