Revert "package/pkg-utils.mk: break hardlinks in global {TARGET, HOST}_DIR on per-package build"

Commit 21d52e52d8 (package/pkg-utils.mk: break hardlinks in global
{TARGET, HOST}_DIR on per-package build) stopped hardlink the source and
destination when rsyncing per-package directory, on the rationale that
modifying files in-place after the rsync would also modify the original
file, and that break foo-rebuild and can cause issues with post-build
scripts.

However, what 21d52e52d8 did not envision, is that copying instead of
hard-linking has two nasty side effects:

  - the size increase for the build directory increase with the number
    of packages and with the depth of th dependency chains for those
    packages: a (relatively small) build that was previously totalling
    ~13GiB in output/, now totals north of 122GiB, an almost 10-time
    increase;

  - the build time increases, as it takes more time to read+write files
    than it takes to create a new link to a file; the same build saw an
    increase of build time from 5min 10s to 7min 30s, a 45% increase.

These regressions are both serious, so revert the change; we'll come up
with a stopgap measure in follow-up commits.

This reverts commit 21d52e52d8.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Herve Codina <herve.codina@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>

package/pkg-utils.mk

@@ -217,7 +217,7 @@ ifeq ($(BR2_PER_PACKAGE_DIRECTORIES),y)
 define per-package-rsync
 	mkdir -p $(3)
 	$(foreach pkg,$(1),\
-		rsync -a \
+		rsync -a --link-dest=$(PER_PACKAGE_DIR)/$(pkg)/$(2)/ \
 			$(PER_PACKAGE_DIR)/$(pkg)/$(2)/ \
 			$(3)$(sep))
 endef