toolchain: sdk: ib: allow external toolchain override

toolchain: sdk: ib: allow external toolchain override

Allow building imagebuilder and/or sdk with dependency on the same
external toolchain as used to build the imagebuilder or sdk (so that ib
and sdk may be built using an external toolchain).

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>
Original patch:
https://lists.openwrt.org/pipermail/openwrt-devel/2016-January/012552.html
[rebased to current main, impvove commit title]
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22089
Signed-off-by: Robert Marko <robimarko@gmail.com>