samsung: misc: add function for setting $dfu_alt_info

samsung: misc: add function for setting $dfu_alt_info

This change introduces new common function:
- set_dfu_alt_info() - put dfu system and bootloader setting
                       into $dfu_alt_info.
functions declaration:
- char *get_dfu_alt_system(void)
- char *get_dfu_alt_boot(void)
- void set_dfu_alt_info(void)
and new config:
- CONFIG_SET_DFU_ALT_INFO

This function can be used for auto setting dfu configuration on boot.
Such feature is useful for multi board support by one u-boot binary.
Each board should define two functions:
- get_dfu_alt_system()
- get_dfu_alt_boot()

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>