Makefile: Do not pass MTD_VERSION from the top Makefile

$(MTD_VERSION) is used in tools/env/Makefile

If you specify a variable at a command line like:
  $ make MTD_VERSION=old env
or specify it thru an envrionment variable like:
  $ export MTD_VERSION=old
  $ make env
it is inherited to the sub-make too.
We do not need to pass it from the top Makefile explicitely.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -1093,7 +1093,7 @@ $(TIMESTAMP_FILE):
 		@cmp -s $@ $@.tmp && rm -f $@.tmp || mv -f $@.tmp $@
 
 easylogo env gdb:
-	$(Q)$(MAKE) $(build)=tools/$@ MTD_VERSION=${MTD_VERSION}
+	$(Q)$(MAKE) $(build)=tools/$@
 
 gdbtools: gdb