Makefile: Add a 'check' target for make

At present we use 'make tests' to run the tests. For many projects
'make check' is more common, so support that as well. Also add some help
to 'make help'.
Signed-off-by: NSimon Glass <sjg@chromium.org>

Makefile

@@ -1698,6 +1698,10 @@ help:
 	@echo  'Configuration targets:'
 	@$(MAKE) -f $(srctree)/scripts/kconfig/Makefile help
 	@echo  ''
+	@echo  'Test targets:'
+	@echo  ''
+	@echo  '  check           - Run all automated tests that use sandbox'
+	@echo  ''
 	@echo  'Other generic targets:'
 	@echo  '  all		  - Build all necessary images depending on configuration'
 	@echo  '  tests		  - Build U-Boot for sandbox and run tests'
@@ -1736,7 +1740,7 @@ help:
 	@echo  'Execute "make" or "make all" to build all targets marked with [*] '
 	@echo  'For further info see the ./README file'
 
-tests:
+tests check:
 	$(srctree)/test/run
 
 # Documentation targets