tools: kwbimage: Refactor line parsing and fix error

The function image_create_config_parse_oneline is pretty complex, and
since more parameters will be added to support secure booting, we
refactor the function to make it more readable.

Also, when a line contained just a keyword without any parameters,
strtok_r returned NULL, which was then indiscriminately fed into atoi,
causing a segfault. To correct this, we add a NULL check before feeding
the extracted token to atoi, and print an error message in case the
token is NULL.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>