Adds support for named rootfs configurations

If the relative or absolute folder doesn't exist then FEX will search in the data folder for a rootFS named the same thing
If the path exists then it is used.

For example if my data folder contains `$HOME/.fex-emu/RootFS/Ubuntu_main/` and I set the RootFS option to `Ubuntu_main`
Then this rootfs will be used. Allows you to easily select a rootfs in that folder without having the full file path.

External/FEXCore/Source/Interface/Config/Config.cpp

@@ -237,7 +237,12 @@ namespace FEXCore::Config {
       }
 
       // Expand relative path to absolute
-      return std::filesystem::absolute(Path);
+      Path = std::filesystem::absolute(Path);
+
+      // Only return if it exists
+      if (std::filesystem::exists(Path)) {
+        return Path;
+      }
     }
     return {};
   }
@@ -263,7 +268,18 @@ namespace FEXCore::Config {
 
     if (FEXCore::Config::Exists(FEXCore::Config::CONFIG_ROOTFS)) {
       FEX_CONFIG_OPT(PathName, ROOTFS);
-      ExpandPathIfExists(FEXCore::Config::CONFIG_ROOTFS, PathName());
+      auto ExpandedString = ExpandPath(PathName());
+      if (!ExpandedString.empty()) {
+        // Adjust the path if it ended up being relative
+        FEXCore::Config::EraseSet(FEXCore::Config::CONFIG_ROOTFS, ExpandedString);
+      }
+      else if (!PathName().empty()) {
+        // If the filesystem doesn't exist then let's see if it exists in the fex-emu folder
+        std::string NamedRootFS = GetDataDirectory() + "RootFS/" + PathName();
+        if (std::filesystem::exists(NamedRootFS)) {
+          FEXCore::Config::EraseSet(FEXCore::Config::CONFIG_ROOTFS, NamedRootFS);
+        }
+      }
     }
     if (FEXCore::Config::Exists(FEXCore::Config::CONFIG_THUNKHOSTLIBS)) {
       FEX_CONFIG_OPT(PathName, THUNKHOSTLIBS);

External/FEXCore/Source/Interface/Config/Config.json

@@ -46,7 +46,14 @@
         "Default": "",
         "ShortArg": "R",
         "Desc": [
-          "Which Root filesystem prefix to use"
+          "Which Root filesystem prefix to use",
+          "This can be a filesystem path",
+          "\teg: ~/RootFS/Debian_x86_64",
+          "Or this can be a name of a rootfs",
+          "If the named rootfs exists in the FEX data folder then it will use that one",
+          "\teg: $HOME/.fex-emu/RootFS/<RootFS name>/",
+          "Or if you have XDG_DATA_HOME the config will search in that directory",
+          "\teg: $XDG_DATA_HOME/.fex-emu/RootFS/<RootFS name>/"
         ]
       },
       "ThunkHostLibs": {