spdx.bbclass: Make use of bb.utils' sha1_file()

The same functionality already exists within bitbake, so avoid
duplicating.

(From OE-Core rev: 978f5a8f16bf5942aad73d761df2a00aeb36339d)

Signed-off-by: Olof Johansson <olofjn@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/spdx.bbclass

@@ -202,13 +202,8 @@ def list_files(dir):
     return
 
 def hash_file(file_name):
-    try:
-        with open(file_name, 'rb') as f:
-            data_string = f.read()
-            sha1 = hash_string(data_string)
-            return sha1
-    except:
-        return None
+    from bb.utils import sha1_file
+    return sha1_file(file_name)
 
 def hash_string(data):
     import hashlib