extmod/modssl_mbedtls: Make SSLSocket.getpeercert() optional.

And only enable this method when the relevant feature is available in
mbedtls.  Otherwise, if mbedtls doesn't support getting the peer
certificate, this method always returns None and it's confusing why it does
that.  It's better to remove the method altogether, so the error trying to
use it is more obvious.

Signed-off-by: Damien George <damien@micropython.org>

tests/multi_net/sslcontext_getpeercert.py

@@ -1,6 +1,7 @@
 # Test creating an SSL connection and getting the peer certificate.
 
 try:
+    import io
     import os
     import socket
     import ssl
@@ -42,6 +43,12 @@ def instance0():
 
 # Client
 def instance1():
+    s_test = ssl.wrap_socket(io.BytesIO(), server_side=True, do_handshake=False)
+    s_test.close()
+    if not hasattr(s_test, "getpeercert"):
+        print("SKIP")
+        raise SystemExit
+
     multitest.next()
     s = socket.socket()
     s.connect(socket.getaddrinfo(IP, PORT)[0][-1])
@@ -49,7 +56,7 @@ def instance1():
     client_ctx.verify_mode = ssl.CERT_REQUIRED
     client_ctx.load_verify_locations(cafile=cafile)
     s = client_ctx.wrap_socket(s, server_hostname="micropython.local")
-    print(s.getpeercert(True))
+    print(s.getpeercert(True).hex())
     s.write(b"client to server")
     print(s.read(16))
     s.close()

tests/multi_net/sslcontext_getpeercert.py.exp

@@ -1,5 +1,5 @@
 --- instance0 ---
 b'client to server'
 --- instance1 ---
-None
+3082058930820371a00302010202141b3da08b15005eea265d0b57b8ba99812ab274cb300d06092a864886f70d01010b05003054310b30090603550406130241553113301106035504080c0a536f6d652d537461746531143012060355040a0c0b4d6963726f507974686f6e311a301806035504030c116d6963726f707974686f6e2e6c6f63616c301e170d3233313131393032323932375a170d3238313131373032323932375a3054310b30090603550406130241553113301106035504080c0a536f6d652d537461746531143012060355040a0c0b4d6963726f507974686f6e311a301806035504030c116d6963726f707974686f6e2e6c6f63616c30820222300d06092a864886f70d01010105000382020f003082020a0282020100deee37780ebca47e0f414ba033ebe692d2bc374a0eb1f42556bf266ad704208116ee0d8b2fd0b518074ad0981c4fd1322de81696ccea838884b06f56d3ebe49cf0561050f6f8ced5f7f4f13d086b28779a9647bbfae6c3f3ad68a5b28ee8a1ceb260d87ea300316599c4dd9f6082f89164b590df8695add518339f6730dec4f05b1ef63548329b0a48823035b23737f3303b56aa251dd8dcf0c20e6c1d291374c185ae657b349c20721c7c01a1b393c96d4c5f2bc8e2dfca7dab896e2fa84dee53d2bb6dbd1056970fa1812315e8ee9d92b3cb93e0b563d274bf07dd79600ef403b91d4ce814418b28cfaeb2b7d8401e64f6d4f39283df3204f2fe01f2fd289f5d2078d9ee2f96b6de1fd4284d9274fa38b0ad9ffcce8ffe66673be2cf304ee1b27c7cacaaf4ca76f1e84419e6e80f540add3e91cd469903e9ceb6bd2b1c33caa59acb5516ce8ac00e73d7a551bb65d39bd6af04411e81c20e6bd474d797a0bcd498e26720bd60ae4f900bb1afa59c7ac7a336273c7734ca5874ea63fb8ec787ab702041442da11a922baf5fbeb9eeea4f9f49cb1f659b561806d2169dbed07c43558c908c94e16491fe1a22cd92b8f33c1184353bdc985c88722f65e48024910f723035c0d33b789928296fb193cec6350884243b00bf51422ad09fb7012bd9cad4716803422be0d111deace913fac8cb2be1e96fa8449068430e5424bd0bd10203010001a3533051301d0603551d0e041604147d392a82ab464936fd7d74226694556a2945fd8d301f0603551d230418301680147d392a82ab464936fd7d74226694556a2945fd8d300f0603551d130101ff040530030101ff300d06092a864886f70d01010b05000382020100ae40c015e3eade8dabc84ee357ac9d694e7cd69ce4a1b265880273d16257119aa72fb2aa8b841e2899bea3e8690146f24d963a37825c93bf745447dc6ab09b5f2947671dca13b1e71f5c3e43011d74cdc688ed1215b3016071ae7235d77f79d7bb81f097bb04a08ccf400717721b29e2ea913eb23614610597deee477ed716db7e8ebe11aed39c7035f48259dfa54d88871c1f67159d52ce11eb111fa00708a7d7081c07fd92d54abbaec7ff1b50ce2f6f358857d2f55d1c7b5aa6dd66b9c3c2e654397e2d5330aca9834ff8fd749ce968c706fe3bb1b8510a379ec1910d7ece0212c34d56a2073fb7f25c88fe298568e448d03ec30b348f7d9a8836390216a6da7a8efed50dfb8c21a8531efc158e7f4398f87af18d1bd2926d08d34364bf5d85e88040dff3d3f1da6268dbc0cafa64f544c065380fa695a8d015b385aed0a1fd9ff1d7c2b28a549e04c1132b421f85a85640acac11c69416859fb9b461eeddffa92ae303b35c7233537077068de558dd02715e25aee976a97879038d2952be0d327892ab2fc78716b0d7aab4b923d5d79905f7f8b6a18c42e466fec62f84b6e5957deae0964dab8436b0e0cd4e08012661bafb9588fbfd7068fd6c08ab79101a4bdfe21d95cd0ee0aad7dd8a3ed128071c0ec2d063dc6dfa63189e51bf5d9259e776d7623f745a73f4e12e5c2b90493de1c6436b339e1400891e3e35c31057
 b'server to client'