build: ipkg: new field Alternatives

It's a list of specs of the following form seprated by commas to
describe alternatives provided by this package

    <prio>:<path>:<altpath>

<path> will be a symbolic link to <altpath> of the highest <prio>

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

include/package-ipkg.mk

@@ -159,6 +159,7 @@ Version: $(VERSION)
 $$(call addfield,Depends,$$(Package/$(1)/DEPENDS)
 )$$(call addfield,Conflicts,$$(call mergelist,$(CONFLICTS))
 )$$(call addfield,Provides,$$(call mergelist,$(PROVIDES))
+)$$(call addfield,Alternatives,$$(call mergelist,$(ALTERNATIVES))
 )$$(call addfield,Source,$(SOURCE)
 )$$(call addfield,License,$$(PKG_LICENSE)
 )$$(call addfield,LicenseFiles,$$(PKG_LICENSE_FILES)