pseudomodules: allow defining them in pkg/PKG/Makefile.include

Officially allow packages to define pseudomodules in their Makefile.include as
done in the libcose package.

makefiles/pseudomodules.inc.mk

@@ -131,3 +131,5 @@ NO_PSEUDOMODULES += periph_common
 PSEUDOMODULES += auto_init_skald
 PSEUDOMODULES += skald_ibeacon
 PSEUDOMODULES += skald_eddystone
+
+# Packages may also add modules to PSEUDOMODULES in their `Makefile.include`.