[FS#938] Netgear AC1200 (R6220): can boot snapshot but can't boot image built from master

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Antonio Quartulli (ordex) 

Attached to Project - LEDE Project
Summary - Netgear AC1200 (R6220): can boot snapshot but can't boot image built from master
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - I can happily boot my R6220 when flashed with the kernel/rootfs provided in the snapshot folder (https://downloads.lede-project.org/snapshots/targets/ramips/mt7621/) however I can't it after having flashed my own kernel/rootfs built out of master with a mostly basic config.

I see the device power led blinking but then I can't get an IP from the LAN port.

Attached is the config I used to build the image.

One or more files have been attached.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=938