linux: ensure buildroot_defconfig is writable

If $(KERNEL_SOURCE_CONFIG) is read-only (eg. because Buildroot's source
dir is), the rm of $(KERNEL_ARCH_PATH)/configs/buildroot_defconfig will
either fail, or prompt the user, both of which we want to avoid.

Make it writable by using $(INSTALL).

Fixes: #4363
Signed-off-by: Nathan Lynch <ntl@pobox.com>
[yann.morin.1998@free.fr: use $(INSTALL) instead of cp, don't 'rm -f']
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/linux.mk

@@ -164,7 +164,7 @@ KERNEL_SOURCE_CONFIG = $(BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE)
 endif
 
 define LINUX_CONFIGURE_CMDS
-	cp $(KERNEL_SOURCE_CONFIG) $(KERNEL_ARCH_PATH)/configs/buildroot_defconfig
+	$(INSTALL) -m 0644 $(KERNEL_SOURCE_CONFIG) $(KERNEL_ARCH_PATH)/configs/buildroot_defconfig
 	$(TARGET_MAKE_ENV) $(MAKE1) $(LINUX_MAKE_FLAGS) -C $(@D) buildroot_defconfig
 	rm $(KERNEL_ARCH_PATH)/configs/buildroot_defconfig
 	$(if $(BR2_arm)$(BR2_armeb),