If kernel config options are not properly set, "make scripts" will not
compile dtc.  Update the unable to find dtc error message to check
the kernel config and give better advice on how to create dtc.

Reword another error message to increase clarity.

Signed-off-by: Frank Rowand <frank.rowand@sonymobile.com>
Signed-off-by: Rob Herring <robh@kernel.org>