Dynamically determine maximum CMD data length depending on SCP

If we're using a Secure Channel Protocol, this will add overhead
in terms of the C-MAC appended to the C-APDU.  This means in turn that
the useable length of the data field shrinks by a certain number of
bytes.

Let's make sure the SCP instances expose an 'overhead' property
of how much overhead they add - and that other commands use this to
determine the maximum command data field length.

Change-Id: I0a081a23efe20c77557600e62b52ba90a401058d

pySim/global_platform/scp.py

@@ -218,6 +218,10 @@ class SCP02(SCP):
                         'seq_counter'/Int16ub, 'card_challenge'/Bytes(6), 'card_cryptogram'/Bytes(8))
     kvn_range = [0x20, 0x2f]
 
+    def __init__(self, *args, **kwargs):
+        self.overhead = 8
+        super().__init__(*args, **kwargs)
+
     def dek_encrypt(self, plaintext:bytes) -> bytes:
         cipher = DES.new(self.card_keys.dek, DES.MODE_ECB)
         return cipher.encrypt(plaintext)
@@ -410,6 +414,7 @@ class SCP03(SCP):
 
     def __init__(self, *args, **kwargs):
         self.s_mode = kwargs.pop('s_mode', 8)
+        self.overhead = self.s_mode
         super().__init__(*args, **kwargs)
 
     def dek_encrypt(self, plaintext:bytes) -> bytes: