olpc_battery: Add support for CHARGE_FULL_DESIGN

Some user space software (read: UPower) uses CHARGE_FULL_DESIGN for internal
calculations. The design capacity of the OLPC batteries is effectively fixed
and only needs to be exported.

Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>
Signed-off-by: Paul Fox <pgf@laptop.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>