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>

utils/checkpackagelib/lib_mk.py

@@ -159,6 +159,7 @@ class TypoInPackageVariable(_CheckFunction):
         "ACLOCAL_DIR",
         "ACLOCAL_HOST_DIR",
         "BR_CCACHE_INITIAL_SETUP",
+        "BR_LIBC",
         "BR_NO_CHECK_HASH_FOR",
         "LINUX_EXTENSIONS",
         "LINUX_POST_PATCH_HOOKS",