make: Implement optional features

Fixes #1876

This PR introduces `FEATURES_OPTIONAL` which can be used to tell the
Make system, that the application would like to use some feature, but
the build should proceed even if the selected board cannot provide the
optional feature.

`make buildtest` and `make info-supported-boards` heed this variable
when examining the list of supported boards.

If a word is present in `FEATURES_REQUIRED` and `FEATURES_OPTIONAL`,
then `FEATURES_OPTIONAL` takes precedence.

Makefile.buildtests

@@ -169,8 +169,16 @@ info-build:
 	@echo 'ELFFILE: $(ELFFILE)'
 	@echo 'HEXFILE: $(HEXFILE)'
 	@echo ''
-	@echo 'FEATURES_REQUIRED: $(sort $(FEATURES_REQUIRED))'
-	@echo 'FEATURES_PROVIDED: $(sort $(FEATURES_PROVIDED))'
+	@echo 'FEATURES_REQUIRED (excl. optional features):'
+	@echo '         $(or $(sort $(filter-out $(FEATURES_OPTIONAL), $(FEATURES_REQUIRED))), -none-)'
+	@echo 'FEATURES_OPTIONAL (strictly "nice to have"):'
+	@echo '         $(or $(sort $(FEATURES_OPTIONAL)), -none-)'
+	@echo 'FEATURES_PROVIDED (by the board):'
+	@echo '         $(or $(sort $(FEATURES_PROVIDED)), -none-)'
+	@echo 'FEATURES_MISSING (incl. optional features):'
+	@echo '         $(or $(sort $(filter-out $(FEATURES_PROVIDED), $(FEATURES_REQUIRED))), -none-)'
+	@echo 'FEATURES_MISSING (only non-optional features):'
+	@echo '         $(or $(sort $(filter-out $(FEATURES_OPTIONAL) $(FEATURES_PROVIDED), $(FEATURES_REQUIRED))), -none-)'
 	@echo ''
 	@echo 'CC:      $(CC)'
 	@echo -e 'CFLAGS:$(patsubst %, \n\t%, $(CFLAGS))'
@@ -215,6 +223,8 @@ info-features-missing:
 info-boards-features-missing:
 	@for f in $(BOARDS_FEATURES_MISSING); do echo $${f}; done | column -t
 
+FEATURES_REQUIRED += $(FEATURES_OPTIONAL)
+
 ifneq (, $(filter info-boards-supported info-boards-features-missing info-build, $(MAKECMDGOALS)))
   FEATURES_PROVIDED_BAK := $(FEATURES_PROVIDED)
 
@@ -224,8 +234,11 @@ ifneq (, $(filter info-boards-supported info-boards-features-missing info-build,
 
     FEATURES_MISSING := $$(filter-out $$(FEATURES_PROVIDED), $$(FEATURES_REQUIRED))
     ifneq (, $${FEATURES_MISSING})
-      BOARDS_WITH_MISSING_FEATURES += ${1}
       BOARDS_FEATURES_MISSING += "${1} $${FEATURES_MISSING}"
+
+      ifneq (, $$(filter-out $$(FEATURES_OPTIONAL), $$(FEATURES_MISSING)))
+        BOARDS_WITH_MISSING_FEATURES += ${1}
+      endif
     endif
   endef
 

Makefile.include

@@ -219,9 +219,9 @@ ifneq (, $(filter all, $(if $(MAKECMDGOALS), $(MAKECMDGOALS), all)))
   endif
 
   # Test if all feature requirements were met by the selected board.
-  ifneq (, $(filter-out $(FEATURES_PROVIDED), $(FEATURES_REQUIRED)))
+  ifneq (, $(filter-out $(FEATURES_PROVIDED) $(FEATURES_OPTIONAL), $(FEATURES_REQUIRED)))
     $(shell $(COLOR_ECHO) "$(COLOR_RED)There are unsatisfied feature requirements:$(COLOR_RESET)"\
-                          "$(filter-out $(FEATURES_PROVIDED), $(FEATURES_REQUIRED))" 1>&2)
+                          "$(sort $(filter-out $(FEATURES_PROVIDED) $(FEATURES_OPTIONAL), $(FEATURES_REQUIRED)))" 1>&2)
     EXPECT_ERRORS := 1
   endif
 

examples/default/Makefile

@@ -36,6 +36,8 @@ USEMODULE += ps
 USEMODULE += vtimer
 USEMODULE += defaulttransceiver
 
+FEATURES_OPTIONAL += transceiver
+
 ifneq (,$(filter msb-430,$(BOARD)))
 	USEMODULE += sht11
 endif