Use platform.machine() when processor() fails.

With CPUARCH being set up automatically from the system via the
platform.processor() function, on some machines an empty string
is returned and the script refuses to build any images.

According to the documentation for the platform module, this can
be an issue with some processors, that either return nothing
or return the same string as with platform.machine().

This PR adds the option to obtain the value from platform.machine()
if the first (and preferred) method returns nothing.

createhdds.py

@@ -43,6 +43,11 @@ from six.moves.urllib.request import urlopen
 # directory.
 SCRIPTDIR = os.path.abspath(os.path.dirname(sys.argv[0]))
 CPUARCH = platform.processor()
+# Some processors or platforms return an empty string in
+# previous step and the script then ends with an error
+# saying "Won't create X image on  host."
+# If that happens, let's read the value from the
+# platform.machine() instead.
 if not CPUARCH:
     CPUARCH = platform.machine()
 logger = logging.getLogger('createhdds')
@@ -157,7 +162,7 @@ class GuestfsImage(object):
                 gpt_type = part.get("gpt_type", None)
                 if gpt_type:
                     gfs.part_set_gpt_type(disk, partnum, gpt_type)
-                # format the partition    
+                # format the partition
                 gfs.mkfs(part['filesystem'], partname, label=part.get('label'))
             # do file 'writes' (create a file with a given string as
             # its content)
@@ -308,7 +313,7 @@ class VirtInstallImage(object):
         # build Workstation images out of Everything
         if variant == 'Workstation':
             variant = 'Everything'
-        
+
         try:
             # this is almost complex enough to need fedfind but not
             # quite, I think. also fedfind can't find the 'transient'