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/include/env_default.h b/include/env_default.h
index 4e461c8..7c9c00a 100644
--- a/include/env_default.h
+++ b/include/env_default.h
@@ -62,7 +62,7 @@
 #ifdef	CONFIG_SYS_DISABLE_AUTOLOAD
 	"autoload=0\0"
 #endif
-#ifdef	CONFIG_PREBOOT
+#ifdef	CONFIG_PREBOOT_DEFINED
 	"preboot="	CONFIG_PREBOOT			"\0"
 #endif
 #ifdef	CONFIG_ROOTPATH