feat(brbe): add brbe under feature detection mechanism

This change adds "FEAT_BRBE" to be part of feature detection mechanism.

Previously feature enablement flags were of boolean type, possessing
either 0 or 1. With the introduction of feature detection procedure
we now support three states for feature enablement build flags(0 to 2).

Accordingly, "ENABLE_BRBE_FOR_NS" flag is now modified from boolean
to numeric type to align with the feature detection.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I1eb52863b4afb10b808e2f0b6584a8a210d0f38c