cmake: fix VERBOSE flag defect

There was a typo in Makefile.cmake which caused
the VERBOSE flag to replace rather than append
the command options passed to cmake --build.
This is needed to reenable the scp-code-coverage-v2
jenkins job, which was broken by this change.

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: I0ac824ed70b93f59d1a535122c413fd2836eda5a