devtool: improve modified file preservation to handle directory structures

devtool: improve modified file preservation to handle directory structures

Allow the _add_md5() function to be called with a directory in order to
recursively add the files under it. Additionally, we need to skip
preserving empty directories (since directories aren't listed in the md5
file).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>