Merge branch '2022-01-21-Kconfig-migrations'

- Migrate CONFIG_KEEP_SERVERADDR, CONFIG_UDP_CHECKSUM, CONFIG_TIMESTAMP,
  CONFIG_BOOTP_SERVERIP, CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR,
  CONFIG_SYS_MAX_FLASH_BANKS_DETECT, CONFIG_SYS_MAX_FLASH_BANKS and
  CONFIG_AT91_EFLASH to Kconfig