~ # twrp install /sdcard/SuperSU-v2.79-201612051815.zip 
Installing zip file '/sdcard/SuperSU-v2.79-201612051815.zip'
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
 
*****************
SuperSU installer
*****************
- Mounting /system, /data and rootfs
- Extracting files
- Detecting system compatibility
- System-less mode, boot image support required
- Creating image
- Mounting image
- Creating paths
- Removing old files
- Placing files
 
******************
Boot image patcher
******************
- Finding boot image
--- Boot image: /dev/block/mmcblk0p7
- Extracting ramdisk
- Decompressing ramdisk
- Checking patch status
--- Already patched, attempting to find stock backup
--- Stock backup restored
- Extracting ramdisk
- Decompressing ramdisk
- Checking patch status
- Patching sepolicy
- Adding daemon launcher
- Adding init script
- Creating mount point
- Patching init.*.rc, fstabs, file_contexts, dm-verity
- Creating ramdisk backup
- Compressing ramdisk
- Creating boot image
- Applying hex patches
- Flashing boot image
 
*************************
    IMPORTANT NOTICES    
*************************
First reboot may take a  
few minutes. It can also 
loop a few times. Do not 
interrupt the process!   
*************************
 
- Unmounting /system
- Done !
Done processing script file
~ # uname -a
Linux localhost 3.18.19 #1 SMP PREEMPT Sun Dec 18 01:30:48 CET 2016 armv7l GNU/Linux
~ #

...so back in android, it claims I have no su binary