Makefile: add help for linux-update-defconfig target

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -888,6 +888,8 @@ endif
 ifeq ($(BR2_LINUX_KERNEL),y)
 	@echo '  linux-menuconfig       - Run Linux kernel menuconfig'
 	@echo '  linux-savedefconfig    - Run Linux kernel savedefconfig'
+	@echo '  linux-update-defconfig - Save the Linux configuration to the path specified'
+	@echo '                             by BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE'
 endif
 ifeq ($(BR2_TOOLCHAIN_BUILDROOT),y)
 	@echo '  uclibc-menuconfig      - Run uClibc menuconfig'