Add codenames for the Chromebooks now supported in Libreboot. The names
of the per-board lbmk resources directories are slightly different than
these for naming consistency (e.g. "gru-kevin" instead of "kevin"). The
names in the coreboot src/mainboard are usually of the baseboard
("google/gru" instead of "kevin").
Also add a link to a ChromiumOS docs page listing most if not all
Chromebooks and their codename information among other things.
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>