use mkfs.ext4 instead of genext2fs

switch from genext2fs to mkfs.ext4

Problems with genext2fs:

• slow O(n²) runtime
• need to upgrade ext2 filesystems to ext4 on first boot
• inode size of 128 disallows timestamps beyond 2038 or sub-second precision timestamps

Problems with mkfs.ext4:

• not bit-by-bit reproducible across multiple invocations

The problems with mkfs.ext4 are solved once this PR is merged: https://github.com/tytso/e2fsprogs/pull/118

TODO:

• works in qemu
• works on real hardware
• clean up commit stack