Trying to boot an Ubuntu 19.10 or 20.04 would result in this error:

```
$ sudo ignite run paulgrandperrin/ignite-ubuntu:20.04 --name test -i
...
[    2.295577] EXT4-fs (vda): mounted filesystem with ordered data mode. Opts: (null)
[    2.303564] VFS: Mounted root (ext4 filesystem) on device 254:0.
[    2.313282] devtmpfs: mounted
[    2.319174] Freeing unused decrypted memory: 2040K
[    2.333506] Freeing unused kernel image memory: 1352K
[    2.344741] Write protecting the kernel read-only data: 14336k
[    2.371993] Freeing unused kernel image memory: 2020K
[    2.383185] Freeing unused kernel image memory: 548K
[    2.388744] Run /sbin/init as init process
[    2.406294] Run /etc/init as init process
[    2.410840] Run /bin/init as init process
[    2.415518] Run /bin/sh as init process
[    2.426454] Kernel panic - not syncing: No working init found.  Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance.
[    2.430159] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.19.47 weaveworks#1
[    2.430159] Call Trace:
[    2.430159]  dump_stack+0x63/0x85
[    2.430159]  ? rest_init+0x40/0xb0
[    2.430159]  panic+0xdf/0x239
[    2.430159]  ? putname+0x47/0x50
[    2.430159]  ? rest_init+0xb0/0xb0
[    2.430159]  kernel_init+0xf0/0x110
[    2.430159]  ret_from_fork+0x1f/0x40
[    2.430159] Kernel Offset: disabled
[    2.430159] Rebooting in 1 seconds..
INFO[0004] firecracker exited: status=0
```

Installing busybox and setting `init=/bin/static-sh` as kernel-args correctly spawns a shell but then
trying to execute `/bin/bash` (or any dynamic executable) results in this error:

```
$ sudo ignite run paulgrandperrin/ignite-ubuntu:20.04 --name test --kernel-args "pci=off ip=dhcp console=ttyS0 reboot=k panic=1 root=/dev/vda rw virtio_mmio.device=4K@0xd0000000:5 virtio_mmio.device=4K@0xd0001000:6 init=/bin/static-sh" -i
...
[    2.338103] EXT4-fs (vda): mounted filesystem with ordered data mode. Opts: (null)
[    2.347059] VFS: Mounted root (ext4 filesystem) on device 254:0.
[    2.357809] devtmpfs: mounted
[    2.364615] Freeing unused decrypted memory: 2040K
[    2.379725] Freeing unused kernel image memory: 1352K
[    2.390496] Write protecting the kernel read-only data: 14336k
[    2.419060] Freeing unused kernel image memory: 2020K
[    2.430295] Freeing unused kernel image memory: 548K
[    2.435784] Run /bin/static-sh as init process
[    2.464800] random: fast init done

BusyBox v1.30.1 (Ubuntu 1:1.30.1-4ubuntu5) built-in shell (ash)
Enter 'help' for a list of built-in commands.

/bin/static-sh: can't access tty; job control turned off
/ # stat /bin/bash
  File: /bin/bash
  Size: 1183448         Blocks: 2312       IO Block: 4096   regular file
Device: fe00h/65024d    Inode: 10266       Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-02-17 17:38:12.000000000
Modify: 2019-11-13 00:54:47.000000000
Change: 2020-02-17 17:38:12.000000000

/ # /bin/bash
/bin/static-sh: /bin/bash: not found
```

The "not found" error is not about the executable itself, but about the
dynamic linker.
The dynamic linker address is `/lib64/ld-linux-x86-64.so.2` which is itself
a symlink to `/lib/x86_64-linux-gnu/ld-2.30.so`.

Unfortunatly on the firecracker VM image, /lib/ only contains the `modules`
directory from the `ignite-kernel` container image and nothing else.

This is because on those distributions, `/lib` is supposed to be a
symlink to `usr/lib` but when the `/lib` directory from the kernel is
extracted, it overrides this symlink, breaking dynamic linking.

From the gnu-tar manual, the `--keep-directory-symlink` provides exactly
the semantic we are looking for.

This patch simply adds this flag where the kernel tree is extracted over
the OS tree.