toolchain/crosstool-NG: do not depend on the top-level Buildroot's .config

Previously, the crosstool-NG backend did depend on the top-level
Buildroot's .config to detect changes in the toolchain options,
using a tentatively-clever heuristic, which also included the full
Buildroot's version string to push down to set the components' versions
strings.

In doing so, any commit in the Buildroot tree would imply a complete
rebuild of the toolchain, even in the case the toolchain options did
not change, thus being a large annoyance (to say the least).

As Buildroot never guaranteed that toolchain options would be detected,
even less handled, and that the internal backend does neither detect nor
act on toolchain options changes, and delegate that to the user, there
is no point in individualising the crosstool-NG backend's behaviour.

This reasoning also applies to the depdency on the crosstool-NG's bundled
.config file, too.

So, just drop the not-so-clever heuristic, and just build the toolchain
once, leaving to the user the responsibility to explictly ask Buildroot
to rebuild the toolchain.

Reported-by: "Przemyslaw Wrzos" <przemyslaw.wrzos@calyptech.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: "Przemyslaw Wrzos" <przemyslaw.wrzos@calyptech.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>