Makefile.include: also detect features and blacklist on flash

Now also print warnings if there are missing features or a blacklisted
board when doing 'make flash'

Before, only when doing 'make' or 'make all' the FEATURES_REQUIRED,
BOARD_BLACKLIST/WHITELIST and TOOLCHAINS were checked and a warning was printed.

However as 'flash' triggers 'all' it is a common case to do 'make flash'
directly instead of 'make all flash'. So better also print warnings in
this case.

Makefile.include

@@ -567,7 +567,7 @@ include $(RIOTMAKE)/vars.inc.mk
 include $(RIOTMAKE)/tools/targets.inc.mk
 
 # Warn if the selected board and drivers don't provide all needed features:
-ifneq (, $(filter all, $(if $(MAKECMDGOALS), $(MAKECMDGOALS), all)))
+ifneq (, $(filter all flash, $(if $(MAKECMDGOALS), $(MAKECMDGOALS), all)))
   EXPECT_ERRORS :=
   EXPECT_CONFLICT :=