[libcalamares] Add System::createTargetFile()

- Calamares may need to create files in the target system;
   provide a convenient API for doing so.
 - This is mostly intended for small files with constant contents.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -243,6 +243,57 @@ System::runCommand(
     return ProcessResult(r, output);
 }
 
+QString
+System::createTargetFile( const QString& path, const QByteArray& contents )
+{
+    QString completePath;
+    
+    if ( doChroot() )
+    {
+        Calamares::GlobalStorage* gs = Calamares::JobQueue::instance() ? Calamares::JobQueue::instance()->globalStorage() : nullptr;
+
+        if ( !gs || !gs->contains( "rootMountPoint" ) )
+        {
+            cWarning() << "No rootMountPoint in global storage, cannot create target file" << path;
+            return QString();
+        }
+        
+        completePath = gs->value( "rootMountPoint" ).toString() + '/' + path;
+    }
+    else
+    {
+        completePath = QStringLiteral( "/" ) + path;
+    }
+    
+    QFile f( completePath );
+    if ( f.exists() )
+    {
+        return QString();
+    }
+    
+    QIODevice::OpenMode m =
+#if QT_VERSION >= QT_VERSION_CHECK( 5, 11, 0 )
+        // New flag from Qt 5.11, implies WriteOnly
+        QIODevice::NewOnly |
+#endif
+        QIODevice::WriteOnly | QIODevice::Truncate;
+        
+    if ( !f.open( m ) )
+    {
+        return QString();
+    }
+    
+    if ( f.write( contents ) != contents.size() )
+    {
+        f.close();
+        f.remove();
+        return QString();
+    }
+    
+    f.close();
+    return QFileInfo( f ).canonicalFilePath();
+}
+
 
 QPair<quint64, float>
 System::getTotalMemoryB() const

src/libcalamares/utils/CalamaresUtilsSystem.h

@@ -205,6 +205,20 @@ public:
         return targetEnvOutput( QStringList{ command }, output, workingPath, stdInput, timeoutSec );
     }
 
+    /** @brief Create a (small-ish) file in the target system.
+     * 
+     * @param path Path to the file; this is **always** interpreted
+     *      from the root of the target system (whatever that may be,
+     *      but / in the chroot, or / in OEM modes).
+     * @param contents Actual content of the file.
+     * 
+     * Will not overwrite files. Returns an empty string if the
+     * target file already exists.
+     * 
+     * @return The complete canonical path to the target file, or empty on failure.
+     */
+    QString createTargetFile( const QString& path, const QByteArray& contents );
+    
     /**
      * @brief getTotalMemoryB returns the total main memory, in bytes.
      *