x86: revert default root size back to 256 MB

2 GB is overkill and was only added to allow unlimited ext4 resizing,
which is a pretty rare use case. 256 MB allows resizing up to 256 GB,
which should be good enough for almost all users.

A lot of this is mostly irrelevant anyway, since you can just use
squashfs + ext4 overlay.

Signed-off-by: Felix Fietkau <nbd@nbd.name>