Add dtb support for Build/sysupgrade-tar definition and
sysupgrade-tar.sh script.
This changes are required for updating dtb separately.
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16904
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>