b2/: add "ignore" keyword for substitution rules

This allows us to decide in the project-specific rules how to indicate
DNP/NC/DNS/... parts. In the original BOOM, F1 == NC was hard-coded.

b2/SUBST

@@ -1,10 +1,12 @@
+FN=DNP { ignore }
+
 REF=R[0-9]* {
 	T=R
 	VAL=(#R) { R=$1 }
 	TOL <= 5%
 	FN=*% { TOL<=$$ }
 	break REF
-//	end break again
+//	end break again ignore
 }
 
 /*