@amteza @guranga
numero53 has created patches ready for a pull request for rbr50/rbs50 and srr60/srs60: Netgear Orbi Pro support-booting from MMC
I'm not sure but it may require to modify target/linux/ipq40xx/image/Makefile to add rbs50 board ID/profile
If you have RBS50 (or RBR50) available please test so numero53 can send the pull request 
Btw it seems rbs50 and rbr50 share the same source, so it's likely their partition layouts are the same for the same firmware version (but differs for different version).