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>
diff --git a/configs/tinker-rk3288_defconfig b/configs/tinker-rk3288_defconfig
index fb42b8b..1007d9c 100644
--- a/configs/tinker-rk3288_defconfig
+++ b/configs/tinker-rk3288_defconfig
@@ -17,7 +17,6 @@
 CONFIG_DEBUG_UART_CLOCK=24000000
 CONFIG_DEBUG_UART=y
 # CONFIG_ANDROID_BOOT_IMAGE is not set
-CONFIG_USE_PREBOOT=y
 CONFIG_DEFAULT_FDT_FILE="rk3288-tinker.dtb"
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_PAD_TO=0x7f8000
diff --git a/configs/tinker-s-rk3288_defconfig b/configs/tinker-s-rk3288_defconfig
index f3a3926..47d089f 100644
--- a/configs/tinker-s-rk3288_defconfig
+++ b/configs/tinker-s-rk3288_defconfig
@@ -17,7 +17,6 @@
 CONFIG_DEBUG_UART_CLOCK=24000000
 CONFIG_DEBUG_UART=y
 # CONFIG_ANDROID_BOOT_IMAGE is not set
-CONFIG_USE_PREBOOT=y
 CONFIG_DEFAULT_FDT_FILE="rk3288-tinker-s.dtb"
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_PAD_TO=0x7f8000