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()

Change-Id: I21cb6b5a5f535dbb10b021c92637f1f4537ecd1b
Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Piotr Wilczek <p.wilczek@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
---
Changes v2:
- change config names:
  CONFIG_SET_DFU_BOOT_ALT to CONFIG_DFU_ALT_BOOTLOADER and
  CONFIG_DFU_BOOT_ALT_* to CONFIG_DFU_ALT_BOOT_*
- change function name: set_dfu_boot_alt() to set_dfu_alt_boot()
- remove superfluous ifdefs

Changes v3:
- set $dfu_alt_info at every boot
- new config: CONFIG_SET_DFU_ALT_INFO
- update commit msg

Changes v4:
- set_dfu_alt_info(): add checking the pointer returned by malloc

Changes v5:
- set_dfu_alt_info(): remove malloc and declare cache aligned buffer
- set_dfu_alt_info(): introduce CONFIG_SET_DFU_ALT_BUF_LEN