rockchip: make use of OpenWrt compiled dtbs

rockchip: make use of OpenWrt compiled dtbs

OpenWrt buildroot will compile all dtbs defined in target to
$(KDIR)/image-$(DEVICE_DTS).dtb, so make use of it to allow us
debug and use external dtbs easier without patching kernel Makefile.

This also fixes commit 5c724939c396 which forgot to update DTS_DIR
in KERNEL variable.

Fixes: 5c724939c396 ("rockchip: add DEVICE_DTS_DIR definition")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/20286
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>