run-build.sh: Allow to override build target using BUILD_TARGET envvar

This is useful to debug complex/long-running CI jobs. Indeed, this change
is made to accommodate testing of ECLAIR integration.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ifc0e5c890c08433c5c2940fa6caa862af6173615
1 file changed