linux-google-nyan: switch to -Os and enable xz compression for modules

as reported in #3477 (closed), the current kernel configuration causes an oversized image, which cannot be installed. Was initially tried in !6076 (closed) but did not catch attention of the users.

Tests on actual HW welcome --> @kip_dynamite please go ahead

closes #3477 (closed)

Signed-off-by: Robert Eckelmann longnoserob@postmarketos.org