contrib/rootfs-builder: Support timestamps and xz compression

I'm not sure when the change happened (sometime since ff5e578,
contrib/rootfs-builder: Support multiple architectures, 2017-09-20,
#479), but Gentoo is now using timestamps and xz compression for their
amd64 stage3:

  $ curl http://distfiles.gentoo.org/releases/amd64/autobuilds/latest-stage3.txt
  # Latest as of Fri, 09 Mar 2018 15:00:02 +0000
  # ts=1520607602
  20180308T214502Z/stage3-amd64-20180308T214502Z.tar.xz 188851072
  20180308T214502Z/hardened/stage3-amd64-hardened-20180308T214502Z.tar.xz 174759140
  20180308T214502Z/hardened/stage3-amd64-hardened+nomultilib-20180308T214502Z.tar.xz 167639136
  20180301T214503Z/hardened/stage3-amd64-hardened-selinux-20180301T214503Z.tar.xz 187319664
  20180308T214502Z/stage3-amd64-nomultilib-20180308T214502Z.tar.xz 181866296
  20180303/systemd/stage3-amd64-systemd-20180303.tar.bz2 281873162
  20180225/uclibc/stage3-amd64-uclibc-hardened-20180225.tar.bz2 57147392
  20180225/uclibc/stage3-amd64-uclibc-vanilla-20180225.tar.bz2 144490332
  20180308T214502Z/stage3-x32-20180308T214502Z.tar.xz 205285160
  20180308T214502Z/hardened/stage4-amd64-hardened+minimal-20180308T214502Z.tar.bz2 261776436
  20180308T214502Z/hardened/stage4-amd64-hardened+minimal-nomultilib-20180308T214502Z.tar.bz2 251821245
  20180308T214502Z/stage4-amd64-minimal-20180308T214502Z.tar.bz2 280918565
  20180308T214502Z/stage4-amd64-minimal-nomultilib-20180308T214502Z.tar.bz2 268835860
  20180303/systemd/stage4-amd64-systemd-20180303.tar.bz2 622067712

This commit adapts to that change, allowing us to continue to pull the
vanilla stage3-amd64-20180308T214502Z.tar.xz.

Signed-off-by: W. Trevor King <[email protected]>

contrib/rootfs-builder/get-stage3.sh

@@ -39,9 +39,10 @@ fi
 
 BASE_ARCH_URL="${BASE_ARCH_URL:-${MIRROR}releases/${BASE_ARCH}/autobuilds/}"
 LATEST=$(wget -O - "${BASE_ARCH_URL}latest-stage3.txt")
-DATE=$(echo "${LATEST}" | sed -n "s|/stage3-${STAGE3_ARCH}-[0-9]*[.]tar[.]bz2.*||p")
+DATE=$(echo "${LATEST}" | sed -n "s|/stage3-${STAGE3_ARCH}-[0-9TZ]*[.]tar[.].*||p")
+COMPRESSION=$(echo "${LATEST}" | sed -n "s|${DATE}/stage3-${STAGE3_ARCH}-${DATE}[.]tar[.]\([^ ]*\) .*|\1|p")
 ARCH_URL="${ARCH_URL:-${BASE_ARCH_URL}${DATE}/}"
-STAGE3="${STAGE3:-stage3-${STAGE3_ARCH}-${DATE}.tar.bz2}"
+STAGE3="${STAGE3:-stage3-${STAGE3_ARCH}-${DATE}.tar.${COMPRESSION}}"
 STAGE3_CONTENTS="${STAGE3_CONTENTS:-${STAGE3}.CONTENTS}"
 STAGE3_DIGESTS="${STAGE3_DIGESTS:-${STAGE3}.DIGESTS.asc}"