samples: subsys: shell: devmem_load: fix incorrect use of OVERLAY_CONFIG

Replace OVERLAY_CONFIG with CONF_FILE since the file in question
contains an alternative configuration to the one in prj.conf
as clearly stated in the sample doc
'If you use poll you should also use `prj_poll.conf` instead of
`prj.conf`.'

Signed-off-by: Abderrahmane Jarmouni <[email protected]>

Author: JarmouniA

samples/subsys/shell/devmem_load/README.md

@@ -20,7 +20,7 @@ west flash
 
 Building for boards without UART interrupt support:
 ```bash
-west build -b native_sim -- -DEXTRA_CONF_FILE=prj_poll.conf  samples/subsys/shell/devmem_load
+west build -b native_sim -- -DCONF_FILE=prj_poll.conf  samples/subsys/shell/devmem_load
 ```
 ## Running
 After connecting to the UART console you should see the following output:

samples/subsys/shell/devmem_load/sample.yaml

@@ -10,7 +10,7 @@ tests:
   sample.devmem_load.polled:
     integration_platforms:
       - native_sim
-    extra_args: EXTRA_CONF_FILE="prj_poll.conf"
+    extra_args: CONF_FILE="prj_poll.conf"
   sample.devmem_load.uart.interrupt:
     integration_platforms:
       - frdm_k64f