Add support for D-Link DIR-878 R1

[Igor Developer]

Hello!
I've recently added PR: https://github.com/openwrt/openwrt/pull/9476
with supporting for D-Link DIR-878 R1, all has compiled and works
perfectly fine.

According to this thread
https://forum.openwrt.org/t/add-support-for-dlink-dir-878-r1/118208/3
I've seen that guy didn't finished his work on it, and it was 2 months
ago. But edits about this PR is about 1 min of coding, 5 min to
compile and 5 min to make a PR.

Everything is going nice through Failsafe UI, according to this thread
https://forum.openwrt.org/t/d-link-dir-882-r1-bootloader-fw-image-verification/74834
Compiled firmware works on D-Link DIR-878 R1 (Ru ver.)

Thanks pyckle for his sign-dlink-ru script :D