Fixup for u-boot update on eMMC

config/sources/sunxi_common.inc

@@ -32,6 +32,9 @@ write_uboot_platform()
 
 setup_write_uboot_platform()
 {
+	# This may cause overwriting u-boot for android or other non-Armbian OS installed on eMMC
+	# so don't run this function by default
+	[[ ! -f /var/lib/armbian/allow_uboot_update ]] && return
 	for dev in $(lsblk -d -n -p -o NAME); do
 		if grep -q 'eGON.BT0' <(dd if=$dev bs=32 skip=256 count=1 status=none); then
 			# Assuming that only one device with SPL signature is present