build(refactor): avoid ifdef comparison

During build 'ENABLE_SPE_FOR_NS=0' is a valid build option however
using ifdef would incorrectly translate this as enabled.

Change-Id: I1c516fb68f6e382bb83c578e499cbb86869d9eca
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
1 file changed