file_list: Add function to get entry by its name ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

file_list: Add function to add an entry to the list ...

Add file_list_add_entry() to add a single entry to a file_list. Then
use it in file_list_parse_one() instead of open coding adding a new
entry.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add function to parse a string in dfu format ...

The dfu command parses a string which contains a list of
devices and flags. This format is useful for other users
aswell, so add common helper functions to parse it and
let the dfu command use this format.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>