Fix root_filesystem_config.txt generation

The first line returned by awk is empty. In consequence, when
fs_config computes its mode it will consider this line as a file (no
trailing slash) and affect the default android_files mode which is
0644. The mode for the root directory should be the default
android_dirs mode 0755.
Add a special case in fs_config to consider empty line as a directory.

Change-Id: I9f33f6fcf4be05c31914db898e65c92b0a611518
Signed-off-by: Gaelle Nassiet <gaellex.nassiet@intel.com>