Bug#825110: yaboot vs grub: metadata_csum and 64bit ext4

[Mathieu Malaterre]

Here is the short explanation on why I played with the `pending` tag.
,
Switching from yaboot to grub for d-i is a great news. However I
feared two things, with the current applied patch for #825110 we are
currently restricting users solely for yaboot user (I did not
understand why the switch was done only for 'powerpc' and 'ppc64' was
skipped). This would also need to be reverted once we fully move to
grub only in d-i.