makefiles: app_dirs.inc.mk initial commit

This helper Makefile is supposed to unify finding folders with
applications that can be built.

makefiles/app_dirs.inc.mk

+# fallback so empty RIOTBASE won't lead to "/examples/"
+RIOTBASE ?= .
+
+# 1. use wildcard to find Makefiles
+# 2. use patsubst to drop trailing "/"
+# 3. use patsubst to drop possible leading "./"
+# 4. sort
+APPLICATION_DIRS := $(sort $(patsubst ./%,%,$(patsubst %/,%,$(dir $(wildcard \
+	$(RIOTBASE)/examples/*/Makefile \
+	$(RIOTBASE)/tests/*/Makefile    \
+	)))))
+
+info-applications:
+	@for dir in $(APPLICATION_DIRS); do echo $$dir; done