Improve notes on portability.
Improve notes on portability.

Describe how to make custom scripts portable by checking for
lava-test-case et al.
Describe how to make test definitions more portable by putting
less reliance on the install: steps, particularly repos.

Change-Id: I9b16c7bc4ce52b77a5316551d39f11f4e4074eb7

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server