rockchip: rk3288-firefly: Drop USE_PREBOOT Kconfig option

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

Drop USE_PREBOOT to speed up boot when booting from SD-card or eMMC.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/configs/firefly-rk3288_defconfig b/configs/firefly-rk3288_defconfig
index 6e395f5..661ac06 100644
--- a/configs/firefly-rk3288_defconfig
+++ b/configs/firefly-rk3288_defconfig
@@ -18,7 +18,6 @@
 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-firefly.dtb"
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_PAD_TO=0x7f8000