file: export MAGIC in SDK

Previously, a wrapper is used for file, which adds '--magic-file'
option to it. But other components might use libmagic and in such
case, if there's no MAGIC environent variable set correctly, things
do not work. For example, rpmbuild makes use of libmagic and it
requries MAGIC to be set correctly.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>