sun7i: Set ARMV7_BOOT_SEC_DEFAULT when OLD_SUNXI_KERNEL_COMPAT is set

Old kernels cannot handle booting in non-secure (hyp) mode, so when
OLD_SUNXI_KERNEL_COMPAT is set, also set ARMV7_BOOT_SEC_DEFAULT.

Note that whether to booting secure or non-secure can always be overriden
using the bootm_boot_mode environment variable.

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