Fix usage of CONFIG_PREBOOT

Due to usage of PREBOOT in Kconfig, macro CONFIG_PREBOOT is always defined
when CONFIG_USE_PREBOOT is enabled. In case CONFIG_PREBOOT is not
explicitly enabled it is set to empty C string and therefore
'#ifdef CONFIG_PREBOOT' guard does not work. Fix this issue by introducing
a new Kconfig symbol PREBOOT_DEFINED which cause to define new C macro
CONFIG_PREBOOT_DEFINED only when CONFIG_PREBOOT is really defined.

Change usage of '#ifdef CONFIG_PREBOOT' by '#ifdef CONFIG_USE_PREBOOT' for
code which checks if preboot code would be called and by
'#ifdef CONFIG_PREBOOT_DEFINED' for defining preboot code.

Signed-off-by: Pali Rohár <pali@kernel.org>

board/boundary/nitrogen6x/nitrogen6x.c

@@ -929,7 +929,7 @@ U_BOOT_CMD(
 	"Returns 0 (true) to shell if key is pressed."
 );
 
-#ifdef CONFIG_PREBOOT
+#ifdef CONFIG_USE_PREBOOT
 static char const kbd_magic_prefix[] = "key_magic";
 static char const kbd_command_prefix[] = "key_cmd";
 
@@ -989,7 +989,7 @@ int misc_init_r(void)
 	gpio_request(IMX_GPIO_NR(2, 3), "search");
 	gpio_request(IMX_GPIO_NR(7, 13), "volup");
 	gpio_request(IMX_GPIO_NR(4, 5), "voldown");
-#ifdef CONFIG_PREBOOT
+#ifdef CONFIG_USE_PREBOOT
 	preboot_keys();
 #endif
 

boot/Kconfig

@@ -1492,6 +1492,10 @@ config PREBOOT
 	help
 	  This is the default of "preboot" environment variable.
 
+config PREBOOT_DEFINED
+	bool
+	default y if PREBOOT != ""
+
 config DEFAULT_FDT_FILE
 	string "Default fdt file"
 	help

include/env_default.h

@@ -62,7 +62,7 @@ const char default_environment[] = {
 #ifdef	CONFIG_SYS_DISABLE_AUTOLOAD
 	"autoload=0\0"
 #endif
-#ifdef	CONFIG_PREBOOT
+#ifdef	CONFIG_PREBOOT_DEFINED
 	"preboot="	CONFIG_PREBOOT			"\0"
 #endif
 #ifdef	CONFIG_ROOTPATH