ci: .shippable.yml: fix _make function
ci: .shippable.yml: fix _make function

The _make function is supposed to return an error status in case make
fails. For this to happen, we need to use && between commands, not ;.

Fixes: 5da449eaf73b ("ci: add .shippable.yml")
Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os