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>
diff --git a/board/boundary/nitrogen6x/nitrogen6x.c b/board/boundary/nitrogen6x/nitrogen6x.c
index 83bb445..382c01d 100644
--- a/board/boundary/nitrogen6x/nitrogen6x.c
+++ b/board/boundary/nitrogen6x/nitrogen6x.c
@@ -929,7 +929,7 @@
 	"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 @@
 	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