makefiles/scan-build: Provide a per board scan-build output

Update the SCANBUILD_OUTPUTDIR to be per board.
Also remove un-necessary export and 'CURDIR'.

makefiles/scan-build.inc.mk

@@ -41,7 +41,7 @@ SCANBUILD_ARGS ?= \
   -analyzer-config stable-report-filename=true \
   #
 
-export SCANBUILD_OUTPUTDIR = $(CURDIR)/scan-build/
+SCANBUILD_OUTPUTDIR = scan-build/$(BOARD)
 
 # Find all variables given on the command line and recreate the command.
 CMDVARS := $(strip $(foreach varname, $(SCANBUILD_ENV_VARS), \