sunxi: Add a bootcmd_sunxi_compat to the default environment to boot old kernels

sunxi: Add a bootcmd_sunxi_compat to the default environment to boot old kernels

Add a bootcmd_sunxi_compat value to the default environment when building
with CONFIG_OLD_SUNXI_KERNEL_COMPAT, this way people who occasionally want
to boot an old kernel can do so by simply typing "run bootcmd_sunxi_compat"
rather then needing to have 2 separate setups / sdcards for old and
new kernels.

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