Added PolarSSL DES library, which is necessary for MSCHAP.

Added a README about our PolarSSL included files, clarifying what we did.

Removed crypt(), -lcrypt ( setkey() / encrypt() ) dependencies.

src/netif/ppp/polarssl/README

@@ -0,0 +1,33 @@
+About PolarSSL files into lwIP PPP support
+------------------------------------------
+
+This folder contains some files fetched from the PolarSSL project for
+ciphers and encryption methods we need for lwIP PPP support.
+
+The PolarSSL files were cleaned to contain only the necessary struct
+fields and functions needed for lwIP.
+
+
+The PolarSSL API was not changed at all, so if you are already using
+PolarSSL you can choose to skip the compilation of the included PolarSSL
+library into lwIP:
+
+The following define are available for flexibility:
+
+LWIP_INCLUDED_POLARSSL_MD4_C   ; Use lwIP internal PolarSSL for MD4
+LWIP_INCLUDED_POLARSSL_MD5_C   ; Use lwIP internal PolarSSL for MD5
+LWIP_INCLUDED_POLARSSL_SHA1_C  ; Use lwIP internal PolarSSL for SHA1
+LWIP_INCLUDED_POLARSSL_DES_C   ; Use lwIP internal PolarSSL for DES
+
+If set (=1), the default if required by another enabled PPP feature unless
+explicitely set to 0, using included lwIP PolarSSL.
+
+If clear (=0), using external PolarSSL.
+
+Undefined if not needed.
+
+Beware of the stack requirements which can be a lot larger if you are not
+using our cleaned PolarSSL library.
+
+
+PolarSSL project website: http://polarssl.org/