tools/mkimage: Use proper output parameter in dtc-system call

The system call used by mkimage to run dtc redirects stdout to a
temporary file. This can cause problems on Windows (with a MinGW
cross-compiled version). Using the "-o" dtc parameter avoids
this problem.

Signed-off-by: Stefan Theil <stefan.theil@mixed-mode.de>
Reviewed-by: Tom Rini <trini@konsulko.com>