efi_loader: Set default console colors on efi_cout_clear_screen if needed

Ensures a consistent background color of the whole screen for succeeding
outputs as both demanded by the spec and implemented in EDK2 as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/lib/efi_loader/efi_console.c b/lib/efi_loader/efi_console.c
index 4d08dd3..9316995 100644
--- a/lib/efi_loader/efi_console.c
+++ b/lib/efi_loader/efi_console.c
@@ -489,6 +489,12 @@
 {
 	EFI_ENTRY("%p", this);
 
+	/* Set default colors if not done yet */
+	if (efi_con_mode.attribute == 0) {
+		efi_con_mode.attribute = 0x07;
+		printf(ESC "[0;37;40m");
+	}
+
 	efi_clear_screen();
 
 	return EFI_EXIT(EFI_SUCCESS);