I followd the same thing, however I did not pay attention enought.
I did not format the sd card as you proposed:
/dev/mmcblk0p1 (boot partition ~20mb)
/dev/mmcblk0p2 (sd card root partition ~1.8gb)
I rather used 1 partion and ended not being able to split into /boot on the sd and /cryptdevice on sata, cause ofcourse it expected the fs somewhere else. I would have to:
create initial ramdisk with mkinitramfs -o /b/initrd
create uInitrd image with cd /b ; mkimage -A arm -O linux -T ramdisk -C gzip -a 0 -e 0 -d initrd uInitrd
and
replace /boot/boot.cmd contents with (what you wrote)
compile boot.cmd to boot.scr with mkimage -C none -A arm -T script -d boot.cmd boot.scr
Unfortunately I can't do that, because I cant boot into the same environment anymore...