Windows equivalent to efibootmgr?
On Windows you can use "bcdedit /enum firmware" to list all firmware applications, then"bcdedit /set {fwbootmgr} bootsequence {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}" to set BootNext option.
Don't you have to create UBuntu loader similar to {9b4692db-d6e1-11e6-8040-f733056555ec}?
Basically the ubuntu node should be changed from boot manager to boot loader app.Then have a boot manager at EFI\Boot\bootx64.efi.
Create a {bootmgr} node for boot manager. Set its displayorder to include both the guids above. Then set the default value for {bootmgr} to ubuntu guid. That should make ubuntu item sticky across reboots.