Populate MinimumVersion property

The Minimum Version is now an available D-Bus interface:
```
https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/70110
```
Populate its value if it's being defined.

Tested: If the msl meson options are defined, the D-Bus property is
populated with the value of the defined minimum version:
```
root@witherspoon:~# busctl get-property xyz.openbmc_project.Software.BMC.Updater /xyz/openbmc_project/software xyz.openbmc_project.Software.MinimumVersion MinimumVersion
s "2.15.0"
```

Change-Id: I0f9df0cfd7c06794df836e161b3416094a8535de
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/msl_verify.cpp b/msl_verify.cpp
index c9f8bce..8dd30a0 100644
--- a/msl_verify.cpp
+++ b/msl_verify.cpp
@@ -59,9 +59,7 @@
 bool minimum_ship_level::verify(const std::string& versionManifest)
 {
     //  If there is no msl or mslRegex return upgrade is needed.
-    std::string msl{BMC_MSL};
-    std::string mslRegex{REGEX_BMC_MSL};
-    if (msl.empty() || mslRegex.empty())
+    if (!enabled())
     {
         return true;
     }
@@ -69,6 +67,7 @@
     // Define mslVersion variable and populate in Version format
     // {major,minor,rev} using parse function.
 
+    std::string msl = getMinimumVersion();
     Version mslVersion = {0, 0, 0};
     parse(msl, mslVersion);
 
@@ -104,3 +103,19 @@
 
     return true;
 }
+
+bool minimum_ship_level::enabled()
+{
+    std::string msl = getMinimumVersion();
+    std::string mslRegex{REGEX_BMC_MSL};
+    if (!msl.empty() && !mslRegex.empty())
+    {
+        return true;
+    }
+    return false;
+}
+
+std::string minimum_ship_level::getMinimumVersion()
+{
+    return BMC_MSL;
+}