kbuild: add LEX and YACC variables

Allow users to use their favorite lexer / parser generators.
This is useful for me to test various flex and bison versions.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index c988e46..83209d6 100644
--- a/Makefile
+++ b/Makefile
@@ -377,6 +377,8 @@
 STRIP		= $(CROSS_COMPILE)strip
 OBJCOPY		= $(CROSS_COMPILE)objcopy
 OBJDUMP		= $(CROSS_COMPILE)objdump
+LEX		= flex
+YACC		= bison
 AWK		= awk
 GENKSYMS	= scripts/genksyms/genksyms
 INSTALLKERNEL  := installkernel
@@ -428,7 +430,7 @@
 
 export ARCH SRCARCH CONFIG_SHELL HOSTCC HOSTCFLAGS CROSS_COMPILE AS LD CC
 export CPP AR NM STRIP OBJCOPY OBJDUMP HOSTLDFLAGS HOST_LOADLIBES
-export MAKE AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
+export MAKE LEX YACC AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
 export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS LDFLAGS