kernel: fitimage: support device tree compiler options
This introduces a new variable to set the device tree compiler options while
calling mkimage ('-D' option). By default, this variable is not set but it can
be defined in a configuration file, as following example:
UBOOT_MKIMAGE_DTCOPTS = "-I dts -O dtb -p 2000"
Signed-off-by: Yannick Gicquel <yannick.gicquel@iot.bzh> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
