sunxi: diable Falcon boot on sun4i & sun7i

Unfortunately it turns out that only sun5i accepts larger SPL images,
and many GCC versions procude an SPL larger than 24KB with Falcon boot
enabled, so we have to keep it disabled until a KB can be shaved off
elsewhere.

All sun5i boards have Falcon mode enabled by default.

Some sun4i/sun7i boards have a separate .._Falcon board type to experiment
with Falcon boot support. But you will need to select toolchain carefully
to generate an SPL that is <24KiB, Linaro GCC 4.7 2013.04 is known to work.

    A20-OLinuXino_MICRO_Falcon
    Cubieboard_Falcon
    Cubieboard2_Falcon