Added bootloader backdoor configuration for cc2538DK in contiki-conf.h and selector in startup-gcc.c. Updated Makefile.cc2538dk to support direct uploading. Also updated the platform readme to include the workflow with cc2538-bsl.
Added the cc2538-bsl submodule to the tools dir
Merge pull request #551 from JelmerT/feature-cc2538-bsl

CC2538 serial boot loader python script