demand_paging: use ram backend by default

In order to be functional and avoid linker errors, use a
default setting for BACKING_STORE_CHOICE that is something other
than "this is not implemented".

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

subsys/demand_paging/backing_store/Kconfig

@@ -3,7 +3,7 @@
 
 choice BACKING_STORE_CHOICE
 	prompt "Backing store algorithms"
-	default BACKING_STORE_CUSTOM
+	default BACKING_STORE_RAM
 
 config BACKING_STORE_CUSTOM
 	bool "Custom backing store implementation"