tools/dustElfToBin: Bug fix related to Windows file path and process operation

[tmineno]

I found the python script (tools/dustElfToBin/dustElfToBin.py) failed on Windows due to three reasons.

1. File path encoding
   Original code use a hard-coded location when an environment variable (IAR_ARM_BASE) is not found.
   However, this treatment should be avoided because the IAR location is much varied depending on version, user's configuration, etc.
   Therefore I fixed this code to just raising exception when IAR_ARM_BASE is not found.

2. UAC
   On Windows 7 or later, sometimes executing ielftool.exe is failed due to privilege issue (Popen returned WindowsError: [Error 5] Access is denied).
   To avoid this error, shell should be set to True.

3. ielftool.exe error
   Although ielftool.exe returns error, the script continued running.
   To handle error case, I changed the script to be terminated when ielftool returns non-zero status.