ramips: rt305x: use lzma-loader for ZyXEL Keenetic Lite rev.B

ramips: rt305x: use lzma-loader for ZyXEL Keenetic Lite rev.B

Fixes boot loader LZMA decompression issue,
reported by GitHub user KOLANICH at [0].

The reported LZMA ERROR has date of 2020-07-20, soon after
the device support landed:

Ralink UBoot Version: 3.5.2.4_ZyXEL

....

3: System Boot system code via Flash.
   Image Name:   MIPS OpenWrt Linux-4.14.187
   Created:      2020-07-20   3:39:11 UTC
   Image Type:   MIPS Linux Kernel Image (lzma compressed)
   Data Size:    1472250 Bytes =  1.4 MB
   Load Address: 80000000
   Entry Point:  80000000
   Verifying Checksum ... OK
   Uncompressing Kernel Image ... LZMA ERROR 1 - must RESET board to recover

[0] https://github.com/openwrt/openwrt/commit/fea232ae8feb6af780fd4fa78ebe9231778bf75a#commitcomment-45016560

Fixes: 4dc9ad4af8c921494d20b303b6772fc6b5af3a69 ("ramips: add support for ZyXEL Keenetic Lite Rev.B")
Signed-off-by: Szabolcs Hubai <szab.hu@gmail.com>
(cherry picked from commit dd3c1ad8ee9ee361285cb9142bdcb35bc3a30ac7)