bluetooth: controller: Put ECDH driver in the MPSL work queue by default

Puts the SDC ECDH driver in the MPSL work queue by default, unless
BT_SMP is selected. It will also be put in the MPSL work queue by
default when split host/controller build configurations are used.

It's expected that many use-cases do not use this module,
and that many applications that do use it, do not necessarily need to
allocate a dedicated thread for its processing. Placing the ECDH work
in the MPSL work queue by default saves considerable RAM for those
use-cases.

We estimate that the ECDH driver processing can take time in the dozens
of milliseconds (possibly more) in the worst case.

Signed-off-by: Olivier Lesage <[email protected]>

subsys/bluetooth/controller/Kconfig

@@ -370,6 +370,7 @@ config BT_CTLR_ECDH_STACK_SIZE
 config BT_CTLR_ECDH_IN_MPSL_WORK
 	bool "ECDH processing in MPSL workqueue"
 	depends on BT_CTLR_ECDH_LIB_OBERON
+	default y if !BT_SMP
 	help
 	  Run the ECDH processing in the MPSL workqueue instead of in a
 	  dedicated pre-emptible thread.