expo: Introduce a test mode

Add a test mode for expo which will display useful debugging
information.

Put this feature behind a CONFIG_EXPO_TEST option to avoid code-size
growth. Create a separate C file and a header. Use static inlines to
avoid lots of CONFIG checking in expo.c

Enable this feature for sandbox and the EFI app for now.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

boot/Kconfig

@@ -982,6 +982,16 @@ config EXPO
 	  The expo can be presented in graphics form using a vidconsole, or in
 	  text form on a serial console.
 
+config EXPO_TEST
+	bool "Enable test mode for expo"
+	depends on EXPO
+	default y if SANDBOX || EFI_APP
+	help
+	  Enable test mode for expo. When enabled, expo displays a frame count
+	  in the top-right corner of the display when the 'expotest' environment
+	  variable is set to 1. This is useful for debugging and performance
+	  analysis.
+
 config BOOTMETH_SANDBOX
 	def_bool y
 	depends on SANDBOX