kernel-fitimage: Allow user to select dtb when multiple dtb exists

Allow user to select the default DTB for FIT image when multiple
dtb's exists.

From machine.conf or local.conf user can specify the default dtb
for FIT image as shown below.

FIT_CONF_DEFAULT_DTB = "board-default.dtb"

Also fallback to avaialable dtb when FIT_CONF_DEFAULT_DTB doesn't
exits or empty.

Signed-off-by: Sandeep Gundlupet Raju <sandeep.gundlupet-raju@amd.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>