efilinux: Explain the config file syntax and add an example

The only explanation of the config file syntax is in the commit log
that introduced it, commit 77148a3e4978 ("efilinux: Minimal
configuration file support"), so add a couple of lines to the README
describing it. Also, add an example config file.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>
2 files changed