dtc: improve reproducibility

Makefile override CFLAGS, so -fmacro/debug-prefix-map
optiions was omitted and binaries contains absolute
patch to sources.

Signed-off-by: Oleksiy Obitotskyy <oobitots@cisco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>