Fix Traceback in CMSIS export.
Fix Traceback in CMSIS export.

The traceback was caused when a file without a parent directory (which
is implicitly ".") is added to the resources object. The code assumed
that all files within the resources object would have a parent
directory.

This commit changes the check to account for files that are in the root
directory of a project and have no preceding `./`

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os