Makefile: introduce check-package target

The snippet of code that runs a check-package on all
.mk/.hash/Config.in files is currently only available within
.gitlab-ci.yml, and isn't immediately and easily usable by Buildroot
users. In order to simplify this, this commit introduces a top-level
"check-package" make target that implements the same logic. The
.gitlab-ci.yml file is changed to use "make check-package".

Since this target is oriented towards Buildroot developers, we
intentionally do not clutter the already noisy "make help" text with
this additional make target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

.gitlab-ci.yml.in

@@ -43,8 +43,7 @@ check-flake8:
 
 check-package:
     script:
-        - find . -type f \( -name '*.mk' -o -name '*.hash' -o -name 'Config.*' \)
-            -exec ./utils/check-package {} +
+        - make check-package
 
 .defconfig: &defconfig
     # Running the defconfigs for every push is too much, so limit to