NEWS:
After many shutdowns, tests, logs, get the N8x0 emulated in qemu to get a console, patches and kernel rebuilds, I finally located the "reboot at shutdown" bug, hidden in the arm assembler kernel code of the Diablo kernel. It's triggered by preemption at kernel level and its effects are not clear. In addition of shutting down normally, after applying the fix I couldn't reproduce the mce "screen doesn't turn off" issue, so it could be caused by this bug too.
Surprisingly, I also learned that the Diablo kernel is 2.6.21.0+omap with almost no bugfixes, while stable mainline reached 2.6.21.7 before Diablo was discontinued, so while I was at it, I imported all applicable bugfixes from 2.6.21.7, and then some from 2.6.22.x after the 2.6.21 end of line, making a grand total of 62 bugfixes.
The new feature of this kernel is the BT UART FIFO optimization patch, for people using a2dp.
Everyone is encouraged to test this kernel, specially those who use Bluetooth with headsets or for inet/transfers.

New testing kernel: Deleted