Git: Update .gitignore

- Only ignore build folders that are relative to the .gitignore file
  (the TF-M root directory)
  - The configs/build_type folder matches "build*/" and ignored by IDEs
    like VS code. Fortunately the folder was added before .gitignore
    so it is not ignored by git
- Remove tf-fuzz settings as it has been moved out from TF-M repo

Change-Id: Ibcd473e1ed1a3d9f55a85fa5c7fb3c0aa7f88c28
Signed-off-by: Kevin Peng <kevin.peng@arm.com>

.gitignore

@@ -10,13 +10,9 @@
 .vscode/
 
 # Garbage files
-build*/
-cmake_build*/
+/build*/
+/cmake_build*/
 *.orig
 **/__pycache__
 *.pyc
 
-# TF-Fuzz build artifacts
-*.o
-*_grammar.*
-tfz