buildroot

check-package: enable for toolchain/

The toolchain directory can benefit from this script to prevent common
mistakes when submitting patches.

In order to accomplish this:
Do not ignore anymore files from the toolchain/ directory.
Ignore this symbol:
 - BR_LIBC: defined by the buildroot toolchain, used by gcc-final.mk.

Ignore toolchain/toolchain-external/pkg-toolchain-external.mk as it
declares a package infra and not a package itself.
Ignore toolchain/helpers.mk as it contains only helper functions.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>