nios2: Enable support for fitImage

The uImage format is legacy for years now, enable support for the
fitImage format, which allows combining multiple files (kernel and
dtb) into a single file, offers better protection of the payload
and so on.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>