[three BSP]fix kconfig file syntax error

bsp/gd32450z-eval/Kconfig

@@ -8,7 +8,7 @@ config BSP_DIR
 config RTT_DIR
     string
     option env="RTT_ROOT"
-    default: "../.."
+    default "../.."
     
 # you can change the RTT_ROOT default: "rt-thread"
 # example : default "F:/git_repositories/rt-thread"

bsp/stm32f7-disco/Kconfig

@@ -8,7 +8,7 @@ config BSP_DIR
 config RTT_DIR
     string
     option env="RTT_ROOT"
-    default: "../.."
+    default "../.."
     
 # you can change the RTT_ROOT default: "../.."
 # example : default "F:/git_repositories/rt-thread"

bsp/stm32l476-nucleo/Kconfig

@@ -8,7 +8,7 @@ config BSP_DIR
 config RTT_DIR
     string
     option env="RTT_ROOT"
-    default: "../.."
+    default "../.."
 
 # you can change the RTT_ROOT default: "rt-thread"
 # example : default "F:/git_repositories/rt-thread"