dfu: add CONFIG_DFU_NAME_MAX_SIZE configuration

Add CONFIG_DFU_NAME_MAX_SIZE to change the proper size.
If name is longer than default size, it can do wrong behavior during updating
image. So it need to change the proper maximum size.

This patch is proviced the solution to change value with configuration.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>