Unified cc2538 examples into single cc2538-common placeholder

examples/cc2538-common/Makefile

examples/cc2538-common/Makefile.target

examples/cc2538-common/README.md

examples/cc2538-common/crypto/Makefile

examples/cc2538-common/crypto/Makefile.target

examples/cc2538-common/crypto/cbc-mac-test.c

examples/cc2538-common/crypto/cbc-test.c

examples/cc2538-common/crypto/ccm-test.c

examples/cc2538-common/crypto/ctr-test.c

examples/cc2538-common/crypto/ecb-test.c

examples/cc2538-common/crypto/gcm-test.c

examples/cc2538-common/crypto/sha256-test.c

examples/cc2538-common/mqtt-demo/Makefile

examples/cc2538-common/mqtt-demo/Makefile.target

examples/cc2538-common/mqtt-demo/README.md

examples/cc2538-common/mqtt-demo/mqtt-demo.c

examples/cc2538-common/mqtt-demo/project-conf.h

examples/cc2538-common/pka/Makefile

examples/cc2538-common/pka/Makefile.target

examples/cc2538-common/pka/ecc-ecdh.c