sunxi: Add environment settings to make extlinux.conf booting work

Automatic booting using an extlinux.conf file requires various environment
variables to be set.

Also modify CONFIG_SYS_LOAD_ADDR and CONFIG_STANDALONE_LOAD_ADDR to match
the value chosen for kernel_addr_r, see the added comment for why the new
value is chosen.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
diff --git a/configs/Mini-X_defconfig b/configs/Mini-X_defconfig
index ceb02c5..ea0c786 100644
--- a/configs/Mini-X_defconfig
+++ b/configs/Mini-X_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="MINI_X,SPL,AXP209_POWER,USB_EHCI"
+CONFIG_FTDFILE="sun4i-a10-mini-xplus.dtb"
 +S:CONFIG_ARM=y
 +S:CONFIG_TARGET_SUN4I=y