Improve Xu4 u-boot update logic

config/sources/odroidxu4.conf

@@ -39,6 +39,19 @@ GOVERNOR=ondemand
 
 SERIALCON=ttySAC2
 
+setup_write_uboot_platform()
+{
+	# this will update u-boot on the device rootfs is located on
+	# in case it's a mmc device, otherwise DEVICE will not be changed
+	# and will default to /dev/mmcblk0
+	local tmp=$(cat /proc/cmdline)
+	tmp="${tmp##*root=}"
+	tmp="${tmp%% *}"
+	[[ -n $tmp ]] && local part=$(findfs $tmp 2>/dev/null)
+	[[ -n $part ]] && local dev=$(lsblk -n -o PKNAME $part 2>/dev/null)
+	[[ -n $dev && $dev == mmcblk* ]] && DEVICE="/dev/${dev}"
+}
+
 write_uboot_platform()
 {
 	# handle writing to eMMC on package upgrades