Update INSTALL instructions for Ubunut Linux Distribution

[DerekHarter]

Update INSTALL-xv6 instructions to give some preliminary
help for installing from a Ubuntu Linux distribution.
The commands were tested on a fresh install of an Ubuntu
20.04 distribution, but will probably work on 18 or even
older Ubuntu distributions, and will be similar for many
(most) other distributions. Only big difference from
MacOS instructions is that the TOOLPREFIX needed for
building the xv6 differs for Ubuntu environment.

[DerekHarter]

Hi,

Not sure if you want or or open to pull requests. As I mentioned in commit comment, I had no real problem setting up xv6 on Ubuntu 20.04, only difference from your MacOS instructions is slight difference in the TOOLPREFIX.

I teach undergrad OS at Texas A&M University - Commerce, and have been playing around with xv6, looking for a simple kernel to create projects for students to code for the OS course working on real schedulers, pagers, etc. I have been using Stallings and other OS books in past. I just finished skimming through your text, and really enjoyed it a lot, thank you for making it available. I am planning on beginning a switch over to using it through this Fall and Spring 2021/2022 semesters.

I am probably going to give my students a Vagrant virtual Ubuntu 20.04 or 21.04 vagrant box as required to do their work on. It looks like there is no problem running xv6 on qemu emulator inside of a virtualbox. I like your projects for xv6 SO, as well as all of the other assignments using emulators and writing programs for OS as well and plan to use them also. May be trying to extend or create additional projects, as about 5 would be ideal for my class.

Again, thanks for the text and resources, I have enjoyed them greatly so far.