lib/functions.mk: initial xport()

A few too many downstream makefiles employed the pattern
of "export GLOBAL_VAR := $(VAR)"; macroize that.

features.in/build-vm/config.mk

@@ -1,3 +1,4 @@
 # hooked from ../../lib/sugar.mk
 use/build-vm:
 	@$(call add_feature)
+	@$(call xport,ROOTPW)

features.in/cleanup/config.mk

@@ -1,5 +1,6 @@
 use/cleanup:
 	@$(call add_feature)
+	@$(call xport,CLEANUP_PACKAGES)
 
 use/cleanup/installer: use/cleanup
 	@$(call add,CLEANUP_PACKAGES,'installer-*')

features.in/install2/config.mk

@@ -5,6 +5,7 @@ use/install2: use/stage2 sub/stage2/install2 use/metadata use/cleanup/installer
 	@$(call add,INSTALL2_PACKAGES,branding-$$(BRANDING)-alterator)
 	@$(call add,BASE_PACKAGES,branding-$$(BRANDING)-release)
 	@$(call add,BASE_LISTS,$(call tags,basesystem))
+	@$(call xport,BASE_BOOTLOADER)
 
 use/install2/net: use/install2
 	@$(call add,INSTALL2_PACKAGES,curl)

features.in/install2/install2/stage2cfg.mk

@@ -8,9 +8,6 @@ MKI_PACK_RESULTS = squash:altinst
 # also removed in a cleanup hook but to spare a few cycles...
 HSH_EXCLUDE_DOCS = 1
 
-# pass downstream
-GLOBAL_BASE_BOOTLOADER := $(BASE_BOOTLOADER)
-
 debug::
 	@echo "** install2: IMAGE_PACKAGES: $(IMAGE_PACKAGES)"
 	@echo "** install2: IMAGE_PACKAGES_REGEXP: $(IMAGE_PACKAGES_REGEXP)"

lib/functions.mk

@@ -36,6 +36,14 @@ define try_body
 printf '%s ?= %s\n' '$(1)' '$(2)' >> "$(CONFIG)"; }
 endef
 
+# xport() requests a variable to be exported to the scripts
+xport = $(and $(1),$(xport_body))
+define xport_body
+{ $(log_body); \
+v='$(1:GLOBAL_%=%)'; \
+printf 'export GLOBAL_%s = $$(%s)\n' "$$v" "$$v" >> "$(CONFIG)"; }
+endef
+
 # if the rule being executed isn't logged yet, log it
 define log_body
 { [ -s "$(CONFIG)" ] && \