Nokia RX-51: Remove CONFIG_PREBOOT from defconfig

CONFIG_PREBOOT just cause putting "preboot=CONFIG_PREBOOT" into env list.
Value CONFIG_PREBOOT="run preboot" in defconfig is just nonsense and does
not do anything useful (it is infinite recursion). Config file for this
board already contains default preboot= env variable with correct value,
which has higher priority than CONFIG_PREBOOT and this is reason why
nonsense CONFIG_PREBOOT is ignored.

Remove nonsense and unused CONFIG_PREBOOT from nokia_rx51_defconfig file.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/configs/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 6c3a95c..3b559ab 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -24,7 +24,6 @@
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="run sdboot;run emmcboot;run attachboot;echo"
 CONFIG_USE_PREBOOT=y
-CONFIG_PREBOOT="run preboot"
 # CONFIG_SYS_DEVICE_NULLDEV is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Nokia RX-51 # "