ramips: use lzma-loader for I-O DATA mt7621 devices

ramips: use lzma-loader for I-O DATA mt7621 devices

The official sysupgrade images for I-O DATA devices manufactured by
MSTC (MitraStar Technology Corp.) cannot be booted normally and the
kernel panics after switching to kernel 5.4.

This commit fixes the issue by using lzma-loader.

Note:
  These devices use Z-LOADER to read the kernel from NAND flash and boot
  it. Z-LOADER cannot load and start plain lzma-loader, so additional
  lzma-compression is needed.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Co-developed-by: Yanase Yuki <dev@zpc.sakura.ne.jp>
Signed-off-by: Yanase Yuki <dev@zpc.sakura.ne.jp>
Tested-by: Yanase Yuki <dev@zpc.sakura.ne.jp> [wn-ax2033gr]