fs/ext2: Remove support for auto-calculation of rootfs size

Auto-calculation of the rootfs size cannot be done in a reliable way
as it depends on the host filesystem and is broken on non ext4 host
(see bugs [1] [2]). So let the user specify the size he wants for his
rootfs.

[1] https://bugs.busybox.net/show_bug.cgi?id=8831
[2] https://bugs.busybox.net/show_bug.cgi?id=9496



Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: as suggested by Arnout, use 60 MB as the default size instead
of 64 MB.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>