Revert "image-fdt: boot_get_fdt() return value when no DTB exists"

It has been found that this change breaks the case of an appended device
tree file, so for the problem in question some other solution must be
found.

This reverts commit c6150aaf2f2745141a7c2ceded58d7efbfeace7d.

Reported-by: Bill Pringlemeir <bpringlemeir@nbsps.com>
Reported-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Confirmed-by: Bill Pringlemeir <bpringlemeir@nbsps.com>
Signed-off-by: Tom Rini <trini@ti.com>