Attempt to change to root dir before executing python script

When inside a git repo, attempt to change to the root dir
before running python script. Prevents a bunch of errors
when mktargets.sh is executed from the wrong dir.
Merge pull request #588 from varunbpatil/root_dir

Attempt to change to root dir before executing python script
Remove an unnecessary 'cd' in the mktargets.sh script

There's no need to return to the original directory at the end of
a script - the current working directory within the subshell that
executes the script doesn't affect the working directory in the
calling shell.
Merge pull request #589 from mstorsjo/simplify-script

Remove an unnecessary 'cd' in the mktargets.sh script