boot_fit: Move to build/tasks/

Now we're going to have two tasks (dtimage.mk and boot_fit.mk), so let's
keep them in one place.

Change-Id: Ib51ba9c22084f06bd4fedaead2e10976223f29a4
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
1 file changed