To help people who are stuck, here's the operations I need to do at each boot :
When in the grub chooser (hold shift while booting if it does not show up), edit the boot configuration :
go the to line which says :
linux /vmlinuz-2.6.38-5-generic root=/dev/<your-root-device> ro crashkernel=384M-2G:64M,2G-:128M quiet splash vt.handoff=7
add at the end : break=bottom and remove the splash option.
It should read :
linux /vmlinuz-2.6.38-5-generic root=/dev/<your-root-device> ro crashkernel=384M-2G:64M,2G-:128M quiet vt.handoff=7 break=bottom
and then hit ctrl-x to boot.
Once at the initramfs prompt do :
chroot /root /bin/bash
getty -8 -n -l /bin/bash 38440 tty8&
getty -8 -n -l /bin/bash 38440 tty6&
the upstart should be starting now. And you should have 2 shells available on console 6 and 8 (alt+f6, alt+f8).
When the mountall get stucks, go to one of the console and do :
initctl stop mountall
The command you be stuck (if not you're lucky). Then hit <ctrl-z> to get back the shell prompt then:
kill -9 %1
initctl start mountall
Do a loop of initctl stop mountall and start mountall until all of the fs are mounted and the upstart daemon resume it's boot.
My $0.02 or 0.02€ contrib.