env: Provide a work-around for unquoting fdtfile

env: Provide a work-around for unquoting fdtfile

Some boards use a CONFIG option to specify the value of this variable.
This is normally handled by efi_get_distro_fdt_name() but in the case
of sunxi this does not work, since 'soc' is sunxi, but the files are
in the allwinner directory.

Provide a work-around for this particular case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>