makefiles/riotgen: fix colors and behav. for undef. targets

When executing `make generate-Makefile.ci` in the base directory,
the make system would try to call `riotgen` with `Makefile.ci`,
which does not work. Likewise with any other target like
`make generate-bogus`. The pattern rule for the prerequisites was
not evaluated by make, therefore the check did not work as
intended.

Author: crasbe

makefiles/tools/riotgen.inc.mk

@@ -1,17 +1,21 @@
-.PHONY: riotgen-installed generate-%
+.PHONY: riotgen-installed _generate-%
 
--include makefiles/color.inc.mk
+RIOTMAKE=$(RIOTBASE)/makefiles
+
+-include $(RIOTMAKE)/utils/ansi.mk
+-include $(RIOTMAKE)/color.inc.mk
 
 riotgen-installed:
 	@command -v riotgen > /dev/null 2>&1 || \
 	{ $(COLOR_ECHO) \
-	"$(COLOR_RED)'riotgen' command is not available \
-	please consider installing it from \
-	https://pypi.python.org/pypi/riotgen$(COLOR_RESET)"; \
+	"$(COLOR_RED)'riotgen' command is not available.\n\
+	Please consider installing it with 'pip install riotgen' or download \
+	it from https://pypi.python.org/pypi/riotgen.$(COLOR_RESET)"; \
 	exit 1; }
 
 GENERATORS = board driver example module pkg test
-$(addprefix generate-,$(GENERATORS)): generate-%
+.SECONDEXPANSION:
+$(addprefix generate-,$(GENERATORS)): $$(addprefix _,$$@)
 
-generate-%: riotgen-installed
+_generate-%: riotgen-installed
 	@RIOTBASE=$(CURDIR) riotgen $* -i