uboot-tools: fix libyaml linker error

uboot-tools: fix libyaml linker error

When we cross build uboot-tools, the dtc tool is still compiled for
the host. Therefore, we should not attempt to link the OpenWrt system
libraries. CPU architecture mismatch can lead to build errors.

Fixes: https://github.com/openwrt/openwrt/issues/22924
Fixes: 55925650aabb ("uboot-tools: update to v2026.04")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/22927
Signed-off-by: Robert Marko <robimarko@gmail.com>