dtc: Switch to building and using our own dtc unless provided

This makes us act like the Linux Kernel does and allow for dtc to be
provided externally but otherwise we use the version of dtc that is
included in the sources.  This in turn means that we can drop the
checkdtc logic.  We select DTC in the cases where we will need the dtc
tool provided.

Signed-off-by: Tom Rini <trini@konsulko.com>