target/imagebuilder: use multi-thread support for xz compression

This speeds up the packing of the imagebuilder a lot:

 imagebuilder-T0.tar.xz real 0m25.199s user 2m45.967s sys 0m1.218s
 imagebuilder-T1.tar.xz real 2m02.543s user 2m02.418s sys 0m1.653s
 imagebuilder-T2.tar.xz real 1m03.684s user 1m59.931s sys 0m0.587s
 imagebuilder-T3.tar.xz real 0m48.033s user 2m02.904s sys 0m0.637s
 imagebuilder-T4.tar.xz real 0m38.963s user 2m15.521s sys 0m0.783s
 imagebuilder-T5.tar.xz real 0m37.994s user 2m21.461s sys 0m0.919s
 imagebuilder-T6.tar.xz real 0m39.524s user 2m48.115s sys 0m1.279s
 imagebuilder-T7.tar.xz real 0m34.061s user 2m45.097s sys 0m1.174s
 imagebuilder-T8.tar.xz real 0m27.286s user 2m55.449s sys 0m1.329s
 imagebuilder-T9.tar.xz real 0m25.205s user 2m44.894s sys 0m1.208s

To keep the output reproducible in any case, we enforce a minimum amount
of 2 threads.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
[refactored into reusable NPROC var, more verbose commit message]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

rules.mk

@@ -70,6 +70,7 @@ TARGET_SUFFIX=$(call qstrip,$(CONFIG_TARGET_SUFFIX))
 BUILD_SUFFIX:=$(call qstrip,$(CONFIG_BUILD_SUFFIX))
 SUBDIR:=$(patsubst $(TOPDIR)/%,%,${CURDIR})
 BUILD_SUBDIR:=$(patsubst $(TOPDIR)/%,%,${CURDIR})
+NPROC:=$(shell sysctl -n hw.ncpu 2>/dev/null || nproc)
 export SHELL:=/usr/bin/env bash
 
 IS_PACKAGE_BUILD := $(if $(filter package/%,$(BUILD_SUBDIR)),1)

target/imagebuilder/Makefile

@@ -86,7 +86,7 @@ endif
 	(cd $(PKG_BUILD_DIR); find staging_dir/host/bin/ $(IB_LDIR)/scripts/dtc/ -type f | \
 		$(XARGS) $(SCRIPT_DIR)/bundle-libraries.sh $(PKG_BUILD_DIR)/staging_dir/host)
 	STRIP=sstrip $(SCRIPT_DIR)/rstrip.sh $(PKG_BUILD_DIR)/staging_dir/host/bin/
-	$(TAR) -cf - -C $(BUILD_DIR) $(IB_NAME) | xz -zc -7e > $@
+	$(TAR) -cf - -C $(BUILD_DIR) $(IB_NAME) | xz -T$(if $(filter 1,$(NPROC)),2,0) -zc -7e > $@
 
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