[nrf noup] samples: Fix smp_svr sample for external memory using PM

nrf-squash! [nrf noup] sample/smp_svr: nrf54l15dk test cases
nrf-squash! [nrf noup] samples/**/smp_svr: testcase of nrf54l15pdk with ext-flash DT

Add proper configs to sample.yaml.

Ref: NCSDK-31608

Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

samples/subsys/mgmt/mcumgr/smp_svr/sample.yaml

@@ -174,8 +174,8 @@ tests:
       - DTC_OVERLAY_FILE="boards/nrf54l15dk_nrf54l15_cpuapp_ext_flash.overlay"
       - mcuboot_CONF_FILE="boards/nrf54l15dk_nrf54l15_cpuapp_ext_flash.conf"
       - mcuboot_EXTRA_DTC_OVERLAY_FILE="boards/nrf54l15dk_nrf54l15_cpuapp_ext_flash.overlay"
-    extra_configs:
-      - CONFIG_PM_OVERRIDE_EXTERNAL_DRIVER_CHECK=y
+      - SB_CONFIG_PM_OVERRIDE_EXTERNAL_DRIVER_CHECK=y
+      - SB_CONFIG_PM_EXTERNAL_FLASH_MCUBOOT_SECONDARY=y
     platform_allow:
       - nrf54l15dk/nrf54l15/cpuapp
       - nrf54l15dk/nrf54l10/cpuapp