google-x64cros: drop oem-* packages and compress initramfs

Fixes #3095

Currently google-x64cros cannot be installed to new devices or updated due to initramfs being too large. By removing oem-* packages, kernel modules included in the main initramfs are limited.

A better fix would be including most modules in the initramfs-extra, but that would require some changes to how mkinitfs handles module handling which is out of the scope of this MR currently. (Will most likely be added in a follow-up MR.)