x86: zboot: Create a separate ZBOOT option for zboot logic

Most of the functionality of zboot is contained in the logic which
handles a zimage. Create a separate Kconfig for the logic so that it can
(later) be used without the command itself being enabled.

Enable ZBOOT by default on x86, with the command depending on that. The
existing 'imply' can therefore be removed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 99e59d9..e5ee10a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1053,4 +1053,12 @@
 	  display, memory and build information. It is stored in
 	  struct sysinfo_t after parsing by get_coreboot_info().
 
+config ZBOOT
+	bool "Support the zImage format"
+	default y
+	help
+	  Enable this to support booting the x86-specific zImage format. This
+	  uses a special, binary format containing information about the Linux
+	  format to boot.
+
 endmenu