Since you have Libreboot installed your computer first boots grub that's
located on the flash-chip of your X200. That grub will look for its
configuration in a file called grub.cfg. That file is also located in the
flash chip, in a file system called CBFS.
In the menu-entries of the grub bootloader you will see something like look
for grub.cfg outside of CBFS. Using that option would normally look for
grub.cfg on your hard disks and allow you to boot using that. In this case
modifying the file on your hard disk is an easy way to modify grub's
configurations.
However since you have full-disk encryption, that menu entry will not find
anything. That means that you have to modify the configuration file located
in the flash chip. That means you should read Option B from the links you
posted.
After you know how to modify the grub in the libreboot rom and flash it, you
should have two options:
1) modify the default boot entry to decrypt the hard disk and start the linux
kernel
2) modify the search for grub.cfg... entry to decrypt the hard disk and
then search for grub.cfg from it.
The latter option gives you more freedom to experiment with different grub
configurations without having to reflash each time. However if the only thing
you want to do is to decrypt your disk and boot, then I suggest 1). For that,
follow the last step at
http://libreboot.org/docs/gnulinux/encrypted_trisquel.html on how to create a
suitable grub.cfg.
PS: a third and possibly quite easy option came to my mind. If you want to
experiment without reflashing yet you could try this:
3) the entry look for grub.cfg outside of CBFS will also look for grub.cfg
from usb flash drives. So you could write a grub.cfg on a usb drive, insert
that into your computer when it boots, and choose the look for grub.cfg
outside of CBFS option.
