cmake: Use ${var} instead of @var@

The latter works, but is not widely used or known. CMake with CMP0052
set will not expand it, so replace it with ${var}.

Change-Id: I528e2ba6d4172956df35e3fc8339090d881aca37
Merge topic 'fix-variable-referece' into next

44801406 cmake: Use ${var} instead of @var@