target/linux/advanced: don't hardcode date suffix

Use ROOTFS_SUFFIX instead of hardcoding -$(DATE).

Reported by Lars Lockenvitz.

target/linux/Makefile.in.advanced

@@ -162,7 +162,7 @@ endif
 
 # -----------------------------------------------------------------------------
 # File name for the Linux kernel binary
-LINUX26_KERNEL_NAME:=$(BOARD_NAME)-linux-$(LINUX26_VERSION)-$(DATE)$(KERNEL_EXT)
+LINUX26_KERNEL_NAME:=$(BOARD_NAME)-linux-$(LINUX26_VERSION)$(ROOTFS_SUFFIX)$(KERNEL_EXT)
 
 
 ifeq ($(BOARD_NAME),)