filesystem: add attribute "leftpad" to class LinFixedEF

In some cases, the specs do not specify an absolute record length.
Instead there may be only a minimum record length specified. The card
vendor may then chose to use larger record length at will. This usually
is no problem since the data is usually written from the left and the
remaining bytes are padded at the end (right side) of the data. However
in some rare cases (EF.MSISDN, see also 3GPP TS 51.011, section 10.5.5)
the data must be written right-aligned towards the physical record
length. This means that the data is padded from the left in this case.

To fix this: Let's add a "leftpad" flag to LinFixedEF, which we set to
true in those corner cases. The code that updates the record in
commands.py must then check this flag and padd the data accordingly.

Change-Id: I241d9fd656f9064a3ebb4e8e01a52b6b030f9923
Related: OS#5714

pySim/ts_51_011.py

@@ -181,7 +181,7 @@ class EF_SMS(LinFixedEF):
 # TS 51.011 Section 10.5.5
 class EF_MSISDN(LinFixedEF):
     def __init__(self, fid='6f40', sfid=None, name='EF.MSISDN', desc='MSISDN', **kwargs):
-        super().__init__(fid, sfid=sfid, name=name, desc=desc, rec_len=(15, 34), **kwargs)
+        super().__init__(fid, sfid=sfid, name=name, desc=desc, rec_len=(15, 34), leftpad=True, **kwargs)
 
     def _decode_record_hex(self, raw_hex_data, **kwargs):
         return {'msisdn': dec_msisdn(raw_hex_data)}
@@ -192,8 +192,7 @@ class EF_MSISDN(LinFixedEF):
             encoded_msisdn = enc_msisdn(msisdn)
         else:
             encoded_msisdn = enc_msisdn(msisdn[2], msisdn[0], msisdn[1])
-        alpha_identifier = (list(self.rec_len)[
-                            0] - len(encoded_msisdn) // 2) * "ff"
+        alpha_identifier = (list(self.rec_len)[0] - len(encoded_msisdn) // 2) * "ff"
         return alpha_identifier + encoded_msisdn
 
 # TS 51.011 Section 10.5.6