ramips: fix initramfs image for I-O DATA mt7621 devices

ramips: fix initramfs image for I-O DATA mt7621 devices

This is additional fix of c998ae7f0e9bd51be4935055efbc3834a92698b1.

The sysupgrade image of I-O DATA MT7621 devices manufactured by MSTC
(MitraStar Technology Corp.) faced to the booting issue. This was caused
by imcomplete extraction of large kernel image by U-Boot, and this issue
is occurred in initramfs image after fixing of sysupgrade image.
So, use lzma-loader for initramfs image to fix the issue.

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]