Makefile.include: Fix BUILDRELPATH when RIOTPROJECT is CURDIR.

This fixes building an example that is outside of RIOT with docker and
also fixes building from a release archive.

Makefile.include

@@ -66,7 +66,8 @@ endif
 MAKEOVERRIDES += $(foreach v,$(__DIRECTORY_VARIABLES),$(v)=$($(v)))
 
 # Path to the current directory relative to RIOTPROJECT
-BUILDRELPATH ?= $(CURDIR:$(RIOTPROJECT)/%=%)/
+# trailing '/' is important when RIOTPROJECT == CURDIR
+BUILDRELPATH ?= $(patsubst $(RIOTPROJECT)/%,%,$(CURDIR)/)
 
 # get host operating system
 OS := $(shell uname)