MLBEDSW-10106: Fix bitflags casting issue.

An obscure cast issue occured during another implementation
where attempting to cast the bitflags object causes it to
take the implicit boolean value, rather than the explicit
unsigned cast.

 - This commit prevents such casts by removing the
   implicit boolean cast.
 - Added specific flag-test operator, resulting in
   an actual boolean value.
 - Allowed double-negation semantics (!!) to check
   for non-zero flags.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: Ide581e840a0c848be68bbc3249518ab901ce480b