ArmPkg: Allow SMC/HVC monitor conduit to be specified at runtime

ArmVirtQemu may execute at EL2, in which case monitor calls are
generally made using SMC instructions instead of HVC instructions.

Whether or not this is the case can only be decided at runtime, and so
the associated PCD needs to be settable at runtime, if the platform
definition chooses so. This implies a boolean PCD, given that a feature
PCD is build-time configurable only.

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Sami Mujawar <sami.mujawar@arm.com>

Committed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Doug Flick [MSFT] <doug.edk2@gmail.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmPkg/Library/ArmMonitorLib/ArmMonitorLib.c

@@ -26,7 +26,7 @@ ArmMonitorCall (
   IN OUT ARM_MONITOR_ARGS  *Args
   )
 {
-  if (FeaturePcdGet (PcdMonitorConduitHvc)) {
+  if (PcdGetBool (PcdMonitorConduitHvc)) {
     ArmCallHvc ((ARM_HVC_ARGS *)Args);
   } else {
     ArmCallSmc ((ARM_SMC_ARGS *)Args);