Add *.o and *.a to .gitignore

Executables files should never make it into the repository anyway
and they pollute the output of other commands.

Change-Id: I07c1f927b45b771403965f7e0d545308532938d4

.gitignore

@@ -5,6 +5,8 @@
 *.elf
 *.exe
 *.so
+*.o
+*.a
 
 # android: java build system
 /android/bin