Use functions in alignment.h to get value

Refactor code using get functions from alignment.h to
read values.

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index ba25389..6d54300 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -364,10 +364,7 @@
     cur_time = ctx->serial;
 #endif
 
-    cookie_time = ((unsigned long) cookie[0] << 24) |
-                  ((unsigned long) cookie[1] << 16) |
-                  ((unsigned long) cookie[2] <<  8) |
-                  ((unsigned long) cookie[3]);
+    cookie_time = (unsigned long) MBEDTLS_GET_UINT32_BE(cookie, 0);
 
     if (ctx->timeout != 0 && cur_time - cookie_time > ctx->timeout) {
         ret = -1;