Little blue light staying put and nothing launches, complete system hang.
Why? It is a pure mystery, my friend.

Ongoing porting work. U-Boot has been modified to handle the usual kernel MMC offset changes.

This is a Secondary Program Loader initrd for InkBox. It permits you to switch between Nickel and InkBox at boot by kexec'ing a new kernel from userspace. Only Libra is supported currently, because kexec is actually reliable there.

We're doing it! eMMCs won't stop us!
I've already got a basic initrd booting with kexec and U-Boot supports fastboot, loady as well as command prompt (bootdelay's set to 1 :D)
kexec is very reliable in this kernel, so I think I'm going to make a tiny kernel that will go in the eMMC's unpartitioned space, then make it boot InkBox's kernel (which is too big for the standard allowed space), a bit like U-Boot SPL.
I must admit, 4.1.15 is a breeze after months with 2.6.35.3! Don't worry though, support will continue for all of these older devices.