Add 'GetAFBRootDirPath' function

Add 'GetAFBRootDirPath' function

Add 'GetAFBRootDirPath' function that replace 'GetBindingDirPath'
function as previous 'GetBindingDirPath' was returning
'Binder root directory path' and not 'Binding directory path'.

'GetBindingDirPath' function has been kept to call 'GetAFBRootDirPath'
for compatibility issues.

FYI:
- 'Binding directory path' is the path to the directory that contains
the binding.
- 'Binder root directory path' is the path specified at binder launch
using '--rootdir' option. If no option is specified, binder root
directory is set to the current directory.

BUG-AGL: SPEC-3009

Change-Id: I3f4bb64f1d67558a291481e0987bd93666535add
Signed-off-by: Jonathan Aillet <jonathan.aillet@iot.bzh>