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 8abefc8896)
35413b047c