firefly_rk3288_demo_defconfig: increase ext2 filesystem size

firefly_rk3288_demo_defconfig: increase ext2 filesystem size

Since commit c6bca8cef0 removed autocalculation of the ext2 filesystem
size, the default size is now set to 60MB. However, this is too small
for firefly_rk3288_demo_defconfig. Indeed, the ext2 filesystem contains
the mali blob (90MB) and Qt5 (14MB), totalling to 125MB.

This is a bit too small to fit comfortably on a 128M flash driver, so
increase the size to 250000K (245MB).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>