toolchain-external: add a specific check to avoid Angstrom toolchains

The Angstrom toolchains available at
http://www.angstrom-distribution.org/toolchains/

 are not usable as
external toolchains in Buildroot, because they are not pure toolchains
with just the C library, but instead complete SDKs with many
cross-compiled libraries (Gtk, Qt, glib, neon, sqlite, X.org, and many
more, approximately 200 MB of libraries).

Buildroot cannot use such toolchains, and while this is documented in
our manual, some users still try to do this. Today, one such user came
on the IRC channel, reporting a build problem, which we started
investigating, only to realize after a long time that he was using an
Angstrom toolchain.

To avoid this problem in the future, we explicitly check if the
toolchain is from Angstrom by looking at the vendor part of the tuple
exposed by the toolchain: as soon as it is
<something>-angstrom-<something-else>, we reject the toolchain with an
explanation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>