subprocess: remove Popen in favor of check_output

This begins moving away from the deprecated subprocess calls in an
effort to eventually move to some more global abstraction using the run
convenience method provided in python 3.5.

[ YOCTO #9342 ]

(From OE-Core rev: 0d6b7276003f1afabc6de683f663540327d52bdc)

Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/buildstats.bbclass

@@ -163,7 +163,11 @@ python run_buildstats () {
             bs = os.path.join(bsdir, "build_stats")
             with open(bs, "a") as f:
                 rootfs = d.getVar('IMAGE_ROOTFS', True)
-                rootfs_size = subprocess.Popen(["du", "-sh", rootfs], stdout=subprocess.PIPE).stdout.read()
+                try:
+                    rootfs_size = subprocess.check_output(["du", "-sh", rootfs],
+                            stderr=subprocess.STDOUT).decode('utf-8')
+                except subprocess.CalledProcessError as e:
+                    bb.error("Failed to get rootfs size: %s" % e.output)
                 f.write("Uncompressed Rootfs size: %s" % rootfs_size)
 
     elif isinstance(e, bb.build.TaskFailed):

meta/classes/spdx.bbclass

@@ -219,14 +219,13 @@ def hash_string(data):
 def run_fossology(foss_command, full_spdx):
     import string, re
     import subprocess
-    
-    p = subprocess.Popen(foss_command.split(),
-        stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    foss_output, foss_error = p.communicate()
-    if p.returncode != 0:
+
+    try:
+        foss_output = subprocess.check_output(foss_command.split(),
+                stderr=subprocess.STDOUT).decode('utf-8')
+    except subprocess.CalledProcessError as e:
         return None
 
-    foss_output = unicode(foss_output, "utf-8")
     foss_output = string.replace(foss_output, '\r', '')
 
     # Package info