[U-Boot] Makefile: Only build dtc if needed

[Tom Rini]

On Tue, Nov 13, 2018 at 03:43:07PM -0700, Simon Glass wrote:

> At present U-Boot always builds dtc if CONFIG_OF_CONTROL is defined. This
> is wasteful when the system already has a suitable version available.
> 
> Update the Makefile logic to build dtc only if the version available is
> too old.
> 
> This saves about 3 seconds of CPU time on a clean build for me.
> 
> Signed-off-by: Simon Glass <sjg at chromium.org>
> Reviewed-by: Marek Vasut <marek.vasut at gmail.com>

In the case of new enough host dtc this then fails on sandbox_spl,
chromebook_minnie and others where we need to build
spl/dts/dt-platdata.c with a python import error over lacking _libfdt.

-- 
Tom
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