I guess I finally found out.
1) In firmware/export/config/iaudiox5.h add at the end of the file #define HAVE_DUALBOOT and save
2) ../tools/configure -> target n°30 then type 'B' for (B)ootloader. Then type 'make -j'
3) You get a bootloader.bin file in current directory
4) Add OF firmware file in the current directory. For example fw 2.10e
5) Type something like (adapt according to your file names) ../tools/mkboot -iax5 of_x5_fw_2.10e.bin bootloader.bin dualboot_rockbox_2.10e.bin
6) Put dualboot_rockbox_2.10e.bin (I guess you should rename it to fw_x5.bin) into your FIRMWARE directory on device.
7) Unplug the device, connect the AC charger and the firmware upgrade process should start (once upgraded, the FIRMWARE directory is empty)
The dual-boot bootloader boots the OF when pressing Play (main or remote) for
more than 3 seconds. Hold it a bit longer because the OF also checks buttons.
Short press boots rockbox.