device-samsung-i9100: set getty

Make it possible to get a getty serial shell.

Unfortunately we cannot get to a getty shell unless we also run setenv SWITCH_SEL 3 from the SBL (secondary bootloader) shell. Furthermore, actually getting kernel output requires setting console=ttySAC2,115200 (or telling kernel to ignore CMDLINE passed by bootloader).

See also initial MR where mainline support for i9100 was added, where serial output is also discussed: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1598#how-to-get-serial-output