script: Isolate variables between build sections

Provide the same behavior when building every components and building
select components.

Backport from release branch.

Issue-Id: SCM-5893
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Change-Id: I1e6c18e00068be3fd2eac9e2d8fd2fdcbb19c030