release: Use the ABI of the target release to fetch packages

- Point --rootdir at the installed destdir in the dvd tree.  This
  causes pkg to determine the ABI from the installed destdir instead
  of the host's binaries.  Previously the result was that packages
  for the host's ABI were always downloaded breaking cross-releases
  (e.g. arm64 releases built on an amd64 host included amd64
  packages on the DVD ISO image rather than arm64 packages).  This
  also handles version mismatches, and I tested this by cross-building
  a 15.x arm64 release on a 14.x amd64 host.

- As a result, pkg now does a chdir(3) to the rootdir before running,
  so the -o argument to fetch needs to be updated to be relative to
  rootdir instead of the CWD as make runs.

- Add a new ROOTDIR variable to limit references to "dvd" to one
  place.  Ideally ROOTDIR would be an argument to this script so
  that it didn't really know about the dvd layout at all.

- While here, simplify creation of symlinks by just using a longer
  path to the link name instead of using 'cd' in the shell before
  invoking ln(1).  Also use ln -sf to create the pkg.pkg symlink
  rather than rm + ln.

PR:		278273
Reported by:	gatekeeper <tiago.gasiba@gmail.com>
Reviewed by:	imp, delphij
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D44749

release/scripts/pkg-stage.sh

@@ -8,7 +8,8 @@ export ASSUME_ALWAYS_YES="YES"
 export PKG_DBDIR="/tmp/pkg"
 export PERMISSIVE="YES"
 export REPO_AUTOUPDATE="NO"
-export PKGCMD="/usr/sbin/pkg -d"
+export ROOTDIR="$PWD/dvd"
+export PKGCMD="/usr/sbin/pkg -d --rootdir ${ROOTDIR}"
 export PORTSDIR="${PORTSDIR:-/usr/ports}"
 
 _DVD_PACKAGES="archivers/unzip
@@ -48,14 +49,13 @@ if [ ! -x /usr/local/sbin/pkg ]; then
 	/usr/bin/make -C ${PORTSDIR}/ports-mgmt/pkg install clean
 fi
 
-export DVD_DIR="dvd/packages"
-export PKG_ABI=$(pkg config ABI)
-export PKG_ALTABI=$(pkg config ALTABI 2>/dev/null)
-export PKG_REPODIR="${DVD_DIR}/${PKG_ABI}"
+export PKG_ABI=$(pkg --rootdir ${ROOTDIR} config ABI)
+export PKG_ALTABI=$(pkg --rootdir ${ROOTDIR} config ALTABI 2>/dev/null)
+export PKG_REPODIR="packages/${PKG_ABI}"
 
-/bin/mkdir -p ${PKG_REPODIR}
+/bin/mkdir -p ${ROOTDIR}/${PKG_REPODIR}
 if [ ! -z "${PKG_ALTABI}" ]; then
-	(cd ${DVD_DIR} && ln -s ${PKG_ABI} ${PKG_ALTABI})
+	ln -s ${PKG_ABI} ${ROOTDIR}/packages/${PKG_ALTABI}
 fi
 
 # Ensure the ports listed in _DVD_PACKAGES exist to sanitize the
@@ -84,11 +84,10 @@ ${PKGCMD} fetch -o ${PKG_REPODIR} -d ${DVD_PACKAGES}
 
 # Create the 'Latest/pkg.txz' symlink so 'pkg bootstrap' works
 # using the on-disc packages.
-mkdir -p ${PKG_REPODIR}/Latest
-(cd ${PKG_REPODIR}/Latest && \
-	ln -s ../All/$(${PKGCMD} rquery %n-%v pkg).pkg pkg.pkg)
-(cd ${PKG_REPODIR}/Latest && \
-	rm -f pkg.txz && ln -s pkg.pkg pkg.txz)
+export LATEST_DIR="${ROOTDIR}/${PKG_REPODIR}/Latest"
+mkdir -p ${LATEST_DIR}
+ln -s ../All/$(${PKGCMD} rquery %n-%v pkg).pkg ${LATEST_DIR}/pkg.pkg
+ln -sf pkg.pkg ${LATEST_DIR}/pkg.txz
 
 ${PKGCMD} repo ${PKG_REPODIR}