The current default rootfs size of 256MB in conjunction with 4K blocks
produces an ext4 filesystem which lacks the appropriate amount of backup GDT
entries to support online-resizing.

For x86 targets, increase the default rootfs size to 2048MB which allows
online resizing the filesystem to up to 2TB which is the current theoretical
maximum for LEDE, due to missing GPT support on the root block device.

Note that the filesystem artefact will not occupy 2GB on the build system as
the make_ext4fs utility uses sparse files to generate the filesystem images,
so the actual disk usage is much lower. Furthermore the filesystem images
are gzip compressed, shrinking them to only a few megabytes on the download
server.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Michael Heimpold <mhei@heimpold.de>