Bison is not a prerequirement since it's build as part of the openwrt toolchain.

SVN-Revision: 14900

include/prereq-build.mk

@@ -93,10 +93,6 @@ $(eval $(call RequireCommand,gawk, \
 	Please install GNU awk. \
 ))
 
-$(eval $(call RequireCommand,bison, \
-	Please install GNU bison. \
-))
-
 $(eval $(call RequireCommand,flex, \
 	Please install flex. \
 ))