The first round of security updates for Nougat is ready. It has only been eleven days since the official release of Android 7.0, but keeping with the regular schedule, new factory images and full system OTAs are now available for installation. There are also Marshmallow-based images with the latest security updates for many devices, including the Nexus 5, which will no longer officially receive major version updates. (Note: it looks like the OTA page lists the new updates, but the files aren't linked properly. Just keep checking back.)
Strangely, Android 7.0 images are still not available for the Nexus 6P, Nexus 6, and Nexus 9 LTE. Firmware for these devices was either never posted or only briefly available and then removed shortly thereafter. The reason for the absence is is currently unknown.
These are the new build numbers:
Pixel C – NRD90R
Nexus 6P – MTC20L
Nexus 5X – MTC20K, NRD90S
Nexus 6 – MMB30W, MOB31E
Nexus Player – NRD90R
Nexus 9 LTE – MOB31E
Nexus 9 Wi-Fi – NRD90R
Nexus 5 – MOB31E
Google has not yet posted the Android Security Bulletin for September, but this post will be updated once it's live.
The downloadable factory images and OTA ZIPs are available at the pages linked below. Once Google pushes the source code to AOSP, we'll also post the developer changelog for this round of patches.