memfault: fix fw version reported to Memfault

Unfortunately we can't use C macros in `.conf` files 😢, so we have to
override the fw prefix symbol in a Kconfig file instead. There is
probably a better solution here for default values in the Kconfigs in
NCS, but this is a simple fix for now.

Tested by printing device info on boot:

```
uart:~$ mflt get_device_info
[00:00:06.696,868] <inf> mflt: S/N: 359404230597438
[00:00:06.696,929] <inf> mflt: SW type: app
[00:00:06.696,990] <inf> mflt: SW version: 1.3.0-dev+c0a36e
[00:00:06.697,052] <inf> mflt: HW version: nrf9151dk
```

Signed-off-by: Gillian Minnehan <gillian.minnehan@nordicsemi.no>

Author: gminn

app/overlay-memfault.conf

@@ -17,7 +17,6 @@ CONFIG_MEMFAULT_HTTP_CLIENT_TIMEOUT_MS=30000
 CONFIG_MEMFAULT_EVENT_STORAGE_SIZE=4096
 CONFIG_PM_PARTITION_SIZE_MEMFAULT_STORAGE=0x2A000
 CONFIG_MEMFAULT_COREDUMP_COLLECT_BSS_REGIONS=y
-CONFIG_MEMFAULT_NCS_FW_VERSION_PREFIX="$(APP_VERSION_STRING)"
 
 # Certificate management
 CONFIG_MEMFAULT_ROOT_CERT_STORAGE_NRF9160_MODEM=y

app/src/modules/cloud/Kconfig.cloud

@@ -140,6 +140,14 @@ config APP_CLOUD_MSG_PROCESSING_TIMEOUT_SECONDS
 	  Maximum time allowed for processing a single message in the module's state machine.
 	  The value must be smaller than CONFIG_APP_CLOUD_WATCHDOG_TIMEOUT_SECONDS.
 
+if MEMFAULT
+
+# Set Memfault software_version prefix from the VERSION file in the application
+config MEMFAULT_NCS_FW_VERSION_PREFIX
+	default "$(APPVERSION)+" if "$(APPVERSION)" != ""
+
+endif # MEMFAULT
+
 module = APP_CLOUD
 module-str = Cloud
 source "subsys/logging/Kconfig.template.log_config"