toolchain/crosstool-ng: vendor part of tuple should be 'buildroot'

Since 11017f08, the Crosstool-ng
backend generates toolchains that have a prefix inconsistent with what
Buildroot expects. Buildroot expects a "buildroot" vendor name, while
Crosstool-NG builds toolchain with the "unknown" vendor name.

This is causing build failure such as:

  http://autobuild.buildroot.org/results/15b2c0e50a81b86dd13af684c9254df2bc0df8de/build-end.log



Fix this by changing the vendor part of the tuple used by Crosstool-NG
to "buildroot".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>