The dtc compiler version that adds initial support was available
in 4.11-rc1.  Add the ability to set an additional dtc compiler
flag is needed by overlays.
Signed-off-by: NFrank Rowand <frank.rowand@sony.com>
Acked-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NRob Herring <robh@kernel.org>