Igor is the maintainer of Armbian: http://www.armbian.com/documentation/|
If you use his images then you could simply call a script that does the job (look for "How to install to NAND, SATA & USB?" in the documentation link above).
Unfortunately the last answer is wrong in many regards. Not every image uses 2 partitions, the whole stuff works also when you have just one (like with Armbian), then today you most likely won't use or even find uEnv.txt any more since the stuff is located in boot.cmd which has to be translated to boot.scr after the root= modification.
And then it's a really bad idea to reference partitions by something like sda1 since this numbering might change if you add an USB thumb drive or another disk. Better use PARTUUID/GUID of the partition. It's 2015 and the UUID concept works.