Kbuild: allow building tools without board configuration

Prior to Kbuild, U-Boot could build under tools/ directory
withour configuring for a specific board.

That feature was lost when switching to Kbuild.

This patch revives it again by adding a make target "tools-only".

Usage:
  $ make tools-only

Neither board configuration nor cross compiler are required to
build host tools.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>