Update rootfs-amd64

The Gentoo signature was verified with the following output:
```
$ make rootfs-amd64.tar.gz
gpg --verify downloads/stage3-amd64-systemd-current.tar.bz2.DIGESTS.asc
gpg: Signature made Mon 05 Mar 2018 19:06:38 CET using RSA key ID 2D182910
gpg: Good signature from "Gentoo Linux Release Engineering (Automated Weekly Release Key) <[email protected]>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 13EB BDBE DE7A 1277 5DFD  B1BA BB57 2E0E 2D18 2910
(cd downloads && \
	grep -A1 '^# SHA512 HASH' stage3-amd64-systemd-current.tar.bz2.DIGESTS.asc | \
	grep -v '^--' | \
	sha512sum -c)
stage3-amd64-systemd-20180303.tar.bz2: OK
stage3-amd64-systemd-20180303.tar.bz2.CONTENTS: OK
```

I had the following sha512sum:
```
0febd543bdd9e06ea566a7c07bb36150cca50204cff035b94d3f77aaa4a5aaf307ceff4fe5396a05cb80c043d5298db06e3672f74c0fb6f7539d9e3343ee424f  ./contrib/rootfs-builder/downloads/stage3-amd64-systemd-20180303.tar.bz2
```

Signed-off-by: Alban Crequy <[email protected]>

Author: alban