efi_loader: use efi_create_indexed_name()

Simplify the creation of indexed variable names like 'Boot0000' by using
function efi_create_indexed_name().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>