fs/ext2: generate Y2K38-resilient filesystems

fs/ext2: generate Y2K38-resilient filesystems

When creating a filesystem, mkfs.ext will chose the inode size depending
on the size of the filesystem. Small filesystem get 128-bytes inodes,
while bigger filesystems use 256-byte inodes (inode must be a power of 2
larger or equal to 128, and smaller or equal to the blocksize).

However, 128-byte inodes can't store timestamps past the dreaded
2038-01-19 03:14:07Z deadline, while inodes larger than or equal to 256
do not have the issue.

It turns out that the tipping point to decide whether a filesystem is
small or big, is about around the size of the filesystems we generate
for our runtime tests. This causes the kernel to emit warning like:

    ext2 filesystem being remounted at / supports timestamps until 2038 (0x7fffffff)

We add a new option to our ext2 filesystem, so that user can specify the
size of the inode. That new option defaults to 256 to be resilient to
the Y2K38 problem.

Note: it was already possible for users to explicitly pass the -I
option, through BR2_TARGET_ROOTFS_EXT2_MKFS_OPTIONS. We could have
chosen to extend the existing value with a -I 256, but that is not
satisfactory. Indeed, we do want to ensure that the default is now
Y2K38-OK, even for existing configurations that did not have explicit
setting.

We also pass that new option before the user-specified arbitrary ones,
so that BR2_TARGET_ROOTFS_EXT2_MKFS_OPTIONS still wins (in case -I was
set there).

Signed-off-by: Yann E. MORIN <[email protected]>
[Peter: tweak help text]
Signed-off-by: Peter Korsgaard <[email protected]>