How to set u-boot environment variable

[Robert Latest <boblatest@gmail.com>]

Hi all,

I've perused all u-boot documentation I could find, but nowhere I
found an answer to my (I think) fairly simple question:

Given a bootable SD card, how can I change u-boot's behavior to, for
example, load a different kernel image or use different environment
variables (I'm most desperate to set the device to have a fixed MAC
address)?

I think the environment variables need to be written to a specific
location on the SD card, but u-boot-tools provides no such tool. man
fw_setenv needs a file /etc/env.config, but my system (which is a
finished image I copied from somewhere) doesn't have that.

So, how is it done?

Below is a listing of the /boot partition of my SD card. My system, as
it is, boots the 3.8 kernel.
This begs some additional questions:

- Why is there a 3.10 System.map and not a 3.8 one? Is this file even needed?
- What's the difference between uImage and zImage, and why does one
kernel version have
  a uImage and the other a zImage?
- Is the "dts" file the source that the "dtb" file was created from,
or does it have some
  other purpose?

Thanks,
robert

root@odroid:/boot# ls -l
total 19876
-rw-r--r-- 1 root root  113039 Apr 25  2015 config-3.10.75-84
-rw-r--r-- 1 root root  102534 Mar  4  2015 config-3.8.13.30
-rw-r--r-- 1 root root 4216745 Dec 20 19:33 initrd.img-3.10.75-84
-rw-r--r-- 1 root root   18358 Apr 25  2015 meson8b_odroidc.dtb
-rw-r--r-- 1 root root   36941 Apr 25  2015 meson8b_odroidc.dts
-rw-r--r-- 1 root root 1910532 Apr 25  2015 System.map-3.10.75-84
-rw-r--r-- 1 root root 5468767 Apr 25  2015 uImage-3.10.75-84
-rw-r--r-- 1 root root 3949837 May  4  2015 uInitrd-3.10.75-84
-rw-r--r-- 1 root root 2157425 May  3  2015 uInitrd-3.8.13.30
-rw-r--r-- 1 root root 2352088 May  4  2015 zImage-3.8.13.30
root@odroid:/boot#