ramips: don't reuse KERNEL_DTB for lzma-loader

mt7621 overrides KERNEL_DTB to limit dictionary size, which isn't needed
for our lzma loader.
This saves 15KB on mt7621 devices using uimage-lzma-loader.
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ramips/image/Makefile

@@ -39,7 +39,7 @@ endef
 
 define Device/uimage-lzma-loader
   LOADER_TYPE := bin
-  KERNEL := $(KERNEL_DTB) | loader-kernel | uImage none
+  KERNEL := kernel-bin | append-dtb | lzma | loader-kernel | uImage none
 endef
 
 define Device/seama