bcm53xx: sysupgrade: optimize building UBI image

Use "truncate" to adjust size of existing file instead of "dd" which
required creating a copy. This saves space on tmpfs. It may be as low
as 2.1 MiB when using OpenWrt default user space and way more (20+ MiB)
when flashing vendor firmware.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 8abefc8896e7138a101fbb3d228946c43321503e)

target/linux/bcm53xx/base-files/lib/upgrade/platform.sh

@@ -1,4 +1,4 @@
-RAMFS_COPY_BIN='osafeloader oseama otrx'
+RAMFS_COPY_BIN='osafeloader oseama otrx truncate'
 
 PART_NAME=firmware
 
@@ -281,7 +281,7 @@ platform_do_upgrade_nand_trx() {
 	while [ "$(dd if=$dir/root skip=$ubi_length bs=1 count=4 2>/dev/null)" = "UBI#" ]; do
 		ubi_length=$(($ubi_length + 131072))
 	done
-	dd if=$dir/root of=/tmp/root.ubi bs=131072 count=$((ubi_length / 131072)) 2>/dev/null
+	truncate -s $ubi_length $dir/root
 	[ $? -ne 0 ] && {
 		echo "Failed to prepare new UBI image."
 		return
@@ -289,7 +289,7 @@ platform_do_upgrade_nand_trx() {
 
 	# Flash
 	mtd write /tmp/kernel.trx firmware || exit 1
-	nand_do_upgrade /tmp/root.ubi
+	nand_do_upgrade $dir/root
 }
 
 platform_do_upgrade_nand_seama() {