populate-extfs.sh: fix to handle special file names correctly

populate-extfs.sh: fix to handle special file names correctly

`debugfs' treats spaces and "" specially. So when we are dealing with
file names, great care should be taken to make sure that `debugfs'
recognizes file names correctly.

The basic solution here is:
1. Use quotation marks to handle spaces correctly.
2. Replace "xxx" with ""xxx"" so that debugfs knows that the quotation
   marks are parts of the file name.

[YOCTO #6503]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>