oe-selftest leaves python bytecode around for the selftests it
has found making git status dirty.

Add .gitignore that hides those files from git. While we're at it,
ignore some other common patterns too.

Signed-off-by: Mikko Ylinen <mikko.ylinen@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>