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>