tools: imximage: check return value when open the plugin file

tools: imximage: check return value when open the plugin file

Check return value when open the plugin file.

Coverity report:
** CID 153926:  Error handling issues  (NEGATIVE_RETURNS)
/tools/imximage.c: 542 in copy_plugin_code()

   ifd = open(plugin_file, O_RDONLY|O_BINARY);
>>>  CID 153926:  Error handling issues  (NEGATIVE_RETURNS)
>>> "ifd" is passed to a parameter that cannot be negative.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Reported-by: Coverity (CID: 153926)
Reviewed-by: Tom Rini <trini@konsulko.com>