ramips: mt7621: use lzma-loader for Sercomm NA502s

This fixes a well-known "LZMA ERROR 1" error on Sercomm NA502s, reported
on the OpneWrt forum [0].

[0] https://forum.openwrt.org/t/206640

Signed-off-by: Andreas Böhler <[email protected]>

target/linux/ramips/image/mt7621.mk

@@ -2384,6 +2384,7 @@ TARGET_DEVICES += sercomm_na502
 
 define Device/sercomm_na502s
   $(Device/nand)
+  $(Device/uimage-lzma-loader)
   IMAGE_SIZE := 20971520
   DEVICE_VENDOR := SERCOMM
   DEVICE_MODEL := NA502S