rockchip: rk3288-tinker: Drop USE_PREBOOT Kconfig option

rockchip: rk3288-tinker: Drop USE_PREBOOT Kconfig option

After the migration to use standard boot, storage media should
automatically be initialized in the order listed in boot_targets env.

Drop USE_PREBOOT to speed up boot with ~12 seconds when booting from
e.g. SD-card or eMMC.

Before:

  3,048,599      2,056  main_loop
  3,050,717      2,118  usb_start
 15,070,499 12,019,782  cli_loop

After:

  3,058,244      2,054  main_loop
  3,063,260      5,016  cli_loop

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>