Add run_command_list() to run a list of commands

This new function runs a list of commands separated by semicolon or newline.
We move this out of cmd_source so that it can be used by other code. The
PXE code also uses the new function.

Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Simon Glass <sjg@chromium.org>