Makefile.include: RIOTBOARD as BOARDSDIR fallback

Makefile.include

@@ -89,6 +89,20 @@ __DIRECTORY_VARIABLES += \
 
 BOARDSDIR               := $(abspath $(BOARDSDIR))
 
+# Include Board and CPU configuration, if BOARD is not found in BOARDSDIR
+# e.g. when set by the environment fallback to searching in RIOTBOARD
+ifeq (,$(wildcard $(BOARDSDIR)/$(BOARD)))
+  ifneq ($(RIOTBOARD),$(BOARDSDIR))
+    $(warning The specified board $(BOARD) was not found in $(BOARDSDIR) fallback to RIOTBOARD)
+    BOARDSDIR = $(RIOTBOARD)
+  endif
+  ifeq ($(RIOTBOARD),$(BOARDSDIR))
+    ifeq (,$(wildcard $(BOARDSDIR)/$(BOARD)))
+      $(error The specified board $(BOARD) does not exist.)
+    endif
+  endif
+endif
+
 # Ensure that all directories are set and don't contain spaces.
 ifneq (, $(filter-out 1, $(foreach v,$(__DIRECTORY_VARIABLES),$(words $($(v))))))
   $(info Aborting compilation for your safety.)
@@ -259,9 +273,6 @@ LAZYSPONGE_FLAGS ?= $(if $(filter 1,$(QUIET)),,--verbose)
 ifeq (, $(APPLICATION))
     $(error An application name must be specified as APPLICATION.)
 endif
-ifneq (0,$(shell test -d $(BOARDSDIR)/$(BOARD); echo $$?))
-    $(error The specified board $(BOARD) does not exist.)
-endif
 
 # Use TOOLCHAIN environment variable to select the toolchain to use.
 # Default for macOS: llvm; for other OS: gnu