efi: Create a new CONFIG_EFI

Create a Kconfig which indicates that EFI functionality is in use,
either as a client (EFI app / stub) or provider (EFI loader). This will
make it easier to share code between these two parts of U-Boot

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/efi_loader/Kconfig

@@ -16,6 +16,7 @@ config EFI_LOADER
 	depends on !EFI_STUB || !X86 || X86_64 || EFI_STUB_32BIT
 	depends on !EFI_APP
 	default y if !ARM || SYS_CPU = armv7 || SYS_CPU = armv8
+	select EFI
 	select CHARSET
 	# We need to send DM events, dynamically, in the EFI block driver
 	select DM_EVENT
@@ -610,4 +611,6 @@ endif
 
 source "lib/efi_client/Kconfig"
 
+source "lib/efi/Kconfig"
+
 endmenu