contrib/rootfs-builder: Support timestamps in stage3 dates

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 for some amd64 stages:

  $ 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
  ...

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

contrib/rootfs-builder/get-stage3.sh

@@ -38,7 +38,7 @@ then
 	then
 		die "could not calculate STAGE3_ARCH from ${STAGE3}"
 	fi
-	DATE=$(echo "${STAGE3}" | sed -n "s/stage3-${STAGE3_ARCH}-\([0-9]*\)[.]tar[.].*/\1/p")
+	DATE=$(echo "${STAGE3}" | sed -n "s/stage3-${STAGE3_ARCH}-\([0-9TZ]*\)[.]tar[.].*/\1/p")
 	if test -z "${DATE}"
 	then
 		die "could not calculate DATE from ${STAGE3}"
@@ -72,7 +72,7 @@ then
 	LATEST=$(wget -O - "${BASE_ARCH_URL}latest-stage3.txt")
 	if test -z "${DATE}"
 	then
-		DATE=$(echo "${LATEST}" | sed -n "s|/stage3-${STAGE3_ARCH}-[0-9]*[.]tar.*||p")
+		DATE=$(echo "${LATEST}" | sed -n "s|/stage3-${STAGE3_ARCH}-[0-9TZ]*[.]tar.*||p")
 		if test -z "${DATE}"
 		then
 			die "could not calculate DATE from ${BASE_ARCH_URL}latest-stage3.txt"