kbuild: error out if $(KBUILD_EXTMOD) contains % or :

If the directory path given to KBUILD_EXTMOD (or M=) contains % or :,
the module fails to build.

% is used in pattern rules, and : as the separator of dependencies.

Bail out with a clearer error message.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

Makefile

@@ -129,6 +129,9 @@ endif
 $(if $(word 2, $(KBUILD_EXTMOD)), \
 	$(error building multiple external modules is not supported))
 
+$(foreach x, % :, $(if $(findstring $x, $(KBUILD_EXTMOD)), \
+	$(error module directory path cannot contain '$x')))
+
 # Remove trailing slashes
 ifneq ($(filter %/, $(KBUILD_EXTMOD)),)
 KBUILD_EXTMOD := $(shell dirname $(KBUILD_EXTMOD).)