buildroot

manual: rework using.txt and update common-usage.txt

Consider the second chapter: "starting-up", as a tutorial.
Assuming that, using.txt only contains the very first commands used to get
configure and build its very first target system.

So, the following subsection from using.txt have been to common-usage.txt:
- Offline builds
- Building out-of-tree
- Environment variables

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>