kbuild: export HOSTCXX and HOSTCXXFLAGS

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 866a8c4..a522968 100644
--- a/Makefile
+++ b/Makefile
@@ -363,7 +363,7 @@
 export CONFIG_SHELL HOSTCC HOSTCFLAGS HOSTLDFLAGS CROSS_COMPILE AS LD CC
 export CPP AR NM LDR STRIP OBJCOPY OBJDUMP
 export MAKE AWK PERL
-export DTC CHECK CHECKFLAGS
+export HOSTCXX HOSTCXXFLAGS DTC CHECK CHECKFLAGS
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS UBOOTINCLUDE OBJCOPYFLAGS LDFLAGS
 export KBUILD_CFLAGS KBUILD_AFLAGS