Kconfig: Add missing quotes around default string value

All errors are generated by ./tools/qconfig.py -b -j8 -i whatever.
Error look like this:
warning: style: quotes recommended around default value for string symbol
EFI_VAR_SEED_FILE (defined at lib/efi_loader/Kconfig:130)

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

lib/efi_loader/Kconfig

@@ -129,7 +129,7 @@ if EFI_VARIABLES_PRESEED
 
 config EFI_VAR_SEED_FILE
 	string "File with initial values of non-volatile UEFI variables"
-	default ubootefi.var
+	default "ubootefi.var"
 	help
 	  File with initial values of non-volatile UEFI variables. The file must
 	  be in the same format as the storage in the EFI system partition. The