package/genext2fs: add host variant

genext2fs is built only if the user selected an ext2 root filesystem.

However, some use-cases can't live with the full target/ dir on the
root filesystem, and requires separate partitions (eg. for /usr).
In this case, the user would not select an ext2 root fs in the
Buildrooot menu, and would only generate a tarball of the rootfs.
This tarball would then be used from a post-image script to build
the actual required FSes.

But then, genext2fs is not built, since the ext2 root FS was not
selected.

As for the other filesystem generators, provide a host variant of
genext2fs (genext2fs is already host-package aware, so only needs
adding a Kconfig entry).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>