imx: move imximage header to common location

Move the imximage.h header file to a common location so we can make
use of it from U-Boot too.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>

diff --git a/tools/imximage.h b/include/imximage.h
similarity index 100%
rename from tools/imximage.h
rename to include/imximage.h