sunxi: kconfig: Add %_felconfig rule to enable FEL build of sunxi platforms.

    $ make BOARD_felconfig
is more convenient than
    $ make BOARD_defconfig
    $ echo CONFIG_SPL_FEL=y >> .config
    $ echo CONFIG_SPL_FEL=y >> spl/.config

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>