PPP: Add option to skip FCS table

Option PPP_FCS_TABLE is created which controls if PPPoS FCS calculation
should be done against precalculated table or by using a short algorithm.

Default value is 1, keeps old behaviour.
Setting it to 0 saves around 0.5 kB flash.

src/include/lwip/opt.h

@@ -1703,6 +1703,13 @@
 
 #if PPP_SUPPORT
 
+/**
+ * PPP_FCS_TABLE: Keep a 256*2 byte table to speed up FCS calculation
+ */
+#ifndef PPP_FCS_TABLE
+#define PPP_FCS_TABLE                   1
+#endif
+
 /**
  * PAP_SUPPORT==1: Support PAP.
  */