Allow newlines within command environment vars

Allow newlines within command environment vars

Any environment variable can hold commands to be executed by the 'run'
command. The environment variables preboot, bootcmd and menucmd have
special code for triggering execution in certain circumstances.

We adjust these calls to use run_command_list() instead of run_command().
This change permits these variables to have embedded newlines so that
they work the same as the 'source' command.

Signed-off-by: Simon Glass <sjg@chromium.org>