classes/lib: Update to use python3 command pipeline decoding

In python3, strings are unicode by default. We need to encode/decode
from command pipelines and other places where we interface with the
real world using the correct locales. This patch updates various
call sites to use the correct encoding/decodings.

(From OE-Core rev: bb4685af1bffe17b3aa92a6d21398f38a44ea874)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/toaster.bbclass

@@ -33,6 +33,7 @@ python toaster_layerinfo_dumpdata() {
 
     def _get_git_branch(layer_path):
         branch = subprocess.Popen("git symbolic-ref HEAD 2>/dev/null ", cwd=layer_path, shell=True, stdout=subprocess.PIPE).communicate()[0]
+        branch = branch.decode('utf-8')
         branch = branch.replace('refs/heads/', '').rstrip()
         return branch