ramips: mt7621: use lzma-loader for Sercomm NA502

This fixes a well known "LZMA ERROR 1" error on Sercomm NA502,
reported on the OpenWrt forum. [1]

[1]: https://forum.openwrt.org/t/176942

Signed-off-by: Szabolcs Hubai <[email protected]>

target/linux/ramips/image/mt7621.mk

@@ -2064,6 +2064,7 @@ TARGET_DEVICES += samknows_whitebox-v8
 
 define Device/sercomm_na502
   $(Device/nand)
+  $(Device/uimage-lzma-loader)
   IMAGE_SIZE := 20480k
   DEVICE_VENDOR := SERCOMM
   DEVICE_MODEL := NA502