You're right, it appears that the nice command isn't included in Yocto, but after a little research I think I found a way to re-nice a process, you can use top, it provides an ongoing look at processor activity in real time. within its interactive commands there's the "r" command, it is used to re-nice a process, I haven't tested it yet, but I think it should work.
For more information about it check this link.
Thank you for the replies. Using Busybox worked - it was not even necessary to rebuild it, turns out the 'renice' command is already included in the default version, which was sufficient for what I was trying to accomplish.
It seems reniceing a process through 'top' is not possible - the 'r' command seems to sort the columns instead.
For others' reference, this is how I set the process priority (running the basic Edison Yocto image from Intel):
- Find the desired process ID using 'pidof' or 'top'
- Set the priority with 'busybox renice [niceness] [process ID] ', where [niceness] is an integer between -20 and 20, -20 being highest priority and +20 the lowest.
Looks like process priority is not the culprit in the slow SPI rate, by the way.