Disable metadata_csum when generating ext4 rootfs

Created by: ollieparanoid

Some downstream kernels don't support this, and this recently became the default in e2fsprogs.

@fooforever and @drebrez: unfortunately I could not reproduce the issue with my samsung-i9100, it boots through straight to weston, even with a fresh image I've just created. So I am not sure if it really fixes #1364 (closed). Could one of you test this patch?