samsung: tizen_rpi: Set to slot_a by default when run tfm

samsung: tizen_rpi: Set to slot_a by default when run tfm

In some case, it's resetting to normal mode from download mode.
So it's always running as ramdisk boot mode.

To fix above issue, Set to slot_a by default when run tfm.
"run tfm" means that user tries to flahs entire image.
So it doesn't need that it enters to slot_b mode.

To prevent the overwrite, change the loading address to 0x02500000.
Because scriptaddr is using 0x02400000.

Change-Id: Iad0791484fb30d08d4584763088b80ad05979df1
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>