Make .gitignore more concise and precise

Several of the patterns match files when they should only match
directories, match anywhere in the repo when they should only match at
the root, start with an unneeded `**/`, or a combination of all three.
Fix all such issues as per the .gitignore documentation[1].

[1] https://git-scm.com/docs/gitignore

.gitignore

@@ -15,9 +15,9 @@
 # specific language governing permissions and limitations
 # under the License.
 
-examples/**/proto/Cargo.lock
-**/*.rs.bk
+/examples/**/proto/Cargo.lock
+*.rs.bk
 .user_ta_header.o.d
-**/target
-optee/toolchains
-out
+target/
+/optee/toolchains
+/out