core/legal-info: don't require overriding _LICENSE

Currently, the formatting we impose on the _LICENSE variable requires
that we also use the rarely used := assignment operator, which makes
the _LICENSE variable the only variable that users have to write with
this operator.

This really departs from the simplicity and consistency of using the
append-assignment, which we use for every other variable.

This is because the append-assignment operator surreptiously
introduces a space between the original value and the appended one. But
we can use this knowledge, to match any instance of a space followed by
a comma, and turn it into a single comma.

This allows users to now have a consistent use of the '=' and '+='
operators we use everywhere else in .mk files.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/adding-packages-generic.txt

@@ -429,13 +429,17 @@ not and can not work as people would expect it should:
   the +license+ field of the manifest file for this package. +
   The expected format for this variable must comply with the following rules:
   ** If different parts of the package are released under different
-  licenses, then +comma+ separate licenses (e.g. +`LIBFOO_LICENSE =
-  GPL-2.0+, LGPL-2.1+`+). If there is clear distinction between which
-  component is licensed under what license, then annotate the license
-  with that component, between parenthesis (e.g. +`LIBFOO_LICENSE =
-  GPL-2.0+ (programs), LGPL-2.1+ (libraries)`+).
+     licenses, then +comma+ separate licenses (e.g. +`LIBFOO_LICENSE =
+     GPL-2.0+, LGPL-2.1+`+). If there is clear distinction between which
+     component is licensed under what license, then annotate the license
+     with that component, between parenthesis (e.g. +`LIBFOO_LICENSE =
+     GPL-2.0+ (programs), LGPL-2.1+ (libraries)`+).
+  ** If some licenses are conditioned on a sub-option being enabled, append
+     the conditional licenses with a comma (e.g.: `FOO_LICENSE += , GPL-2.0+
+     (programs)`); the infrastructure will internally remove the space before
+     the comma.
   ** If the package is dual licensed, then separate licenses with the
-  +or+ keyword (e.g. +`LIBFOO_LICENSE = AFL-2.1 or GPL-2.0+`+).
+     +or+ keyword (e.g. +`LIBFOO_LICENSE = AFL-2.1 or GPL-2.0+`+).
 
 * +LIBFOO_LICENSE_FILES+ is a space-separated list of files in the package
   tarball that contain the license(s) under which the package is released.

package/pkg-generic.mk

@@ -1006,7 +1006,7 @@ ifeq ($$($(2)_REDISTRIBUTE),YES)
 endif # redistribute
 
 endif # other packages
-	@$$(call legal-manifest,$$(call UPPERCASE,$(4)),$$($(2)_RAWNAME),$$($(2)_VERSION),$$($(2)_LICENSE),$$($(2)_MANIFEST_LICENSE_FILES),$$($(2)_ACTUAL_SOURCE_TARBALL),$$($(2)_ACTUAL_SOURCE_SITE),$$(call legal-deps,$(1)))
+	@$$(call legal-manifest,$$(call UPPERCASE,$(4)),$$($(2)_RAWNAME),$$($(2)_VERSION),$$(subst $$(space)$$(comma),$$(comma),$$($(2)_LICENSE)),$$($(2)_MANIFEST_LICENSE_FILES),$$($(2)_ACTUAL_SOURCE_TARBALL),$$($(2)_ACTUAL_SOURCE_SITE),$$(call legal-deps,$(1)))
 endif # ifneq ($$(call qstrip,$$($(2)_SOURCE)),)
 	$$(foreach hook,$$($(2)_POST_LEGAL_INFO_HOOKS),$$(call $$(hook))$$(sep))