kernel: skip rebuilding kernel with ROOTFS_INITRAMFS_SEPARATE

With PER_DEVICE_ROOTFS, rebuilding kernel is needed to embed the cpio
image in the kernel image. With ROOTFS_INITRAMFS_SEPARATE, the cpio
image is external hence we can reuse the same kernel image without
rebuilding it.

Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <[email protected]>

include/kernel-defaults.mk

@@ -183,11 +183,13 @@ endif
 	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_LZO),$(STAGING_DIR_HOST)/bin/lzop -9 -f $(if $(2),$(LINUX_DIR)$(2),$(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 $(if $(2),$(LINUX_DIR)$(2),$(KERNEL_BUILD_DIR))/initrd.cpio)
 	$(if $(CONFIG_TARGET_INITRAMFS_COMPRESSION_ZSTD),$(STAGING_DIR_HOST)/bin/zstd -T0 -f -o $(if $(2),$(LINUX_DIR)$(2),$(KERNEL_BUILD_DIR))/initrd.cpio.zstd $(if $(2),$(LINUX_DIR)$(2),$(KERNEL_BUILD_DIR))/initrd.cpio)
-endif
+	$(call Kernel/CopyImage,-initramfs$(2))
+else
 	+$(call locked,$(if $(2),$(CP) $(LINUX_DIR)$(2)/.config* $(LINUX_DIR) && touch $(LINUX_DIR)/.config && )\
 		rm -rf $(LINUX_DIR)/usr/initramfs_data.cpio* $(LINUX_DIR)/.config.prev && \
 		$(KERNEL_MAKE) $(KERNEL_MAKEOPTS_IMAGE) $(if $(KERNELNAME),$(KERNELNAME),all) && \
 		{ $(call Kernel/CopyImage,-initramfs$(2)) },gen-initramfs)
+endif
 endef
 else
 define Kernel/CompileImage/Initramfs