gitignore: change file according to alternation of project structure

.gitignore

@@ -1,7 +1,9 @@
+# Cargo build path and library exclusions
 /target
-/.idea
-/platform/*/.idea
-/rustsbi/.idea
 Cargo.lock
 
+# IntelliJ IDEA directory
+/.idea
+
+# Mac OS X hidden files
 .DS_Store