ramips: mt7621: use seama-lzma-loader for D-Link DIR-860L B1

ramips: mt7621: use seama-lzma-loader for D-Link DIR-860L B1

Fix the LZMA ERROR 1 with a single line of recipe instead of duplicating
"uimage-lzma-loader".

While reviewing my original submission of commit ce1957100411 David
suggested to use $(Device/uimage-lzma-loader), but due to the specific
needs of the vendor bootloader that simple oneliner didn't work.

The new $(Device/seama-lzma-loader) is for those SEAMA capable
bootloaders.

Signed-off-by: Szabolcs Hubai <szab.hu@gmail.com>
(cherry picked from commit 18801f26485e3a0dcb79dc9f9b174aed5821b758)