.travis.yml: simplify build commands
.travis.yml: simplify build commands

Many build commands needlessly set some variables to their default
values: simplify that. Also, reformat commands to make lines shorter,
and introduce variable make="make -j3 -s".

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>
Reviewed-by: Pascal Brand <0b55292d2562685bad4ce52d64f802e8f1cd299e@linaro.org>

pascal-brand-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,matt2048/optee_os,pascal-brand-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os