Custom Flashing to Nexus 7 - product info
You need to set ANDROID_PRODUCT_OUT in /etc/environment.
You may have a folder with boot.img and system.img etc. in, this is the right folder.  

Example: 

ANDROID_PRODUCT_OUT=/path/to/folder/with/images/


After that you can call sudo fastboot flashall -w   

It also works for self builds.