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>

boot/Kconfig

@@ -704,7 +704,7 @@ config SHOW_BOOT_PROGRESS
 	  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