Add default cert store on Fuchsia

Fuchsia isn't POSIX and doesn't have /etc.  This CL adds the
location for the system certificate store on Fuchsia.

Change-Id: I2b48e0e13525a32fa5e2c5c48b8db41d76c26872
Reviewed-on: https://boringssl-review.googlesource.com/19224
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/x509/x509_def.c

@@ -59,7 +59,12 @@
 
 /* TODO(fork): cleanup */
 
+#if defined(OPENSSL_FUCHSIA)
+#define OPENSSLDIR "/system/data/boringssl"
+#else
 #define OPENSSLDIR "/etc/ssl"
+#endif
+
 #define X509_CERT_AREA          OPENSSLDIR
 #define X509_CERT_DIR           OPENSSLDIR "/certs"
 #define X509_CERT_FILE          OPENSSLDIR "/cert.pem"