external-toolchain: run checks even on extracted toolchains

When the mechanism that allows Buildroot to download external
toolchains automatically was added, all the sanity checks on the
external toolchains were not performed. This commit re-enables those
checks that we already do on external toolchains that are not
downloaded/extracted by Buildroot. This makes the toolchain checks
more consistent accross various configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>