kernel: fix initramfs reproducibility

Make sure xz uses at least 2 threads so compression always runs in
multi-threaded mode as the resulting file in single-threaded mode
differs.

Fixes: 29d7461d11 ("kernel: set options to make external initramfs reproducible")
Signed-off-by: Daniel Golle <[email protected]>

include/kernel-defaults.mk

@@ -181,7 +181,7 @@ endif
 	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_GZIP),gzip -n -f -S .gzip -9n $(KERNEL_BUILD_DIR)/initrd.cpio)
 	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZMA),$(STAGING_DIR_HOST)/bin/lzma e -lc1 -lp2 -pb2 $(KERNEL_BUILD_DIR)/initrd.cpio $(KERNEL_BUILD_DIR)/initrd.cpio.lzma)
 # ?	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZO),)
-	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_XZ),$(STAGING_DIR_HOST)/bin/xz -T0 -9 -fz --check=crc32 $(KERNEL_BUILD_DIR)/initrd.cpio)
+	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_XZ),$(STAGING_DIR_HOST)/bin/xz -T$(if $(filter 1,$(NPROC)),2,0) -9 -fz --check=crc32 $(KERNEL_BUILD_DIR)/initrd.cpio)
 # ?	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZ4),)
 	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_ZSTD),$(STAGING_DIR_HOST)/bin/zstd -T0 -f -o $(KERNEL_BUILD_DIR)/initrd.cpio.zstd $(KERNEL_BUILD_DIR)/initrd.cpio)
 endif