Since a TRX image cannot contain a CFE update, I decided to unpack the actual 3602 firmware to see if I could figure out what happened. Looks like Asus inserted the CFE update right inside the firmware code itself, it must probably be done the first time the router gets booted:
Code:merlin@mint-dev ~/fmk/trunk/376-3602-ac68/rootfs/rom/cfe $ ls cfe_1.0.2.0.bin cfe_1.0.2.0_esmt.bin cfe_tmo_2.1.2.2.bin mtd-write
I think I know why they did this tho. Currently, the RT-AC56 and RT-AC68 are running out of space on the firmware partition to add additional features (such as the new DPI engine that will be added later on). I suspect this is how they decided to implement the CFE update that will increase the size of the firmware partition to 64 MB, versus the original 32 MB partition size.
So the 3602 FW does indeed upgrade the bootloader.
Merlin, does this also apply to the RT-AC66U? Thanks.