cmd/elf.c: Add SPDX tag

Based on reading the text of the license comment this appears to be
the BSD-2-Clause license but with an imperfect word match as
BSD-2-Clause was not (as far as I recall) a common license choice at the
time the code was written.

Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>