added option to set secondary ieee address

One of the features of SmartRF Flash Programmer 2 is the ability to set
the secondary (ie. programmable) IEEE address. This commit adds that
same option to cc2538-bsl.py.
Merge pull request #3 from lab11/set-ieee-address

Added option to set secondary ieee address
Return an error code if exiting due to an error so a higher-level Makefile or shell script is aware of the failure.
Merge pull request #7 from hexluthor/error-exit

Return an error code if exiting due to an error
Added version option: this option prints the git tags based version, or if no git repo is available, the fixed version string defined in the script
Merge pull request #8 from JelmerT/feature-versioning

Added versioning system