Move shell script linting into a separate make target

Makefile:
Move shellcheck to a "lint" target and change "check" so that it runs "lint".
This provides a more finer grained control and allows to extend "check" with things beyond just linting.

.gitlab-ci.yml:
Run `make lint` in the lint job.

.gitlab-ci.yml

@@ -10,4 +10,4 @@ default:
 lint:
   stage: build
   script:
-    - make check
+    - make lint

Makefile

@@ -16,7 +16,9 @@ DOC_DIR=$(DESTDIR)/usr/share/doc/archiso
 
 all:
 
-check:
+check: lint
+
+lint:
 	shellcheck -s bash archiso/mkarchiso \
 	                   scripts/run_archiso.sh \
 	                   $(INSTALL_FILES) \
@@ -44,4 +46,4 @@ install-examples:
 install-doc:
 	install -vDm 644 $(DOC_FILES) -t $(DOC_DIR)
 
-.PHONY: check install install-program install-initcpio install-examples install-doc
+.PHONY: check install install-program install-initcpio install-examples install-doc lint