bootstage: Differentiate boot progress kconfig entries

Both U-Boot proper and SPL entries were using the same description.

Fixes: b55881dd ("bootstage: Add SPL support")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index a8d4be2..d3a12be 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -704,7 +704,7 @@
 	  151	common/cmd_nand.c	FIT image format OK
 
 config SPL_SHOW_BOOT_PROGRESS
-	bool "Show boot progress in a board-specific manner"
+	bool "Show boot progress in a board-specific manner in SPL"
 	depends on SPL
 	help
 	  Defining this option allows to add some board-specific code (calling