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
Fix for manual reset problem

Having to press a button or remove a jumper to use the bootloader is a
major hassle. This commit adds support in the BSL script for a
workaround using otherwise unused pins common on FTDI chips: DTR and
RTS. These are both outputs, and since they are not being used for flow
control they can be used to toggle the bootloader pin and the !RESET
line to enable the bootloader.

To have the BSL script automatically invoke the bootloader, pass the -i
flag when calling the script.