[script] Accept labels on lines terminated with CRLF

[script] Accept labels on lines terminated with CRLF

CRLF line terminators are allowed in scripts; the carriage return is
simply interpreted as trailing whitespace and so is ignored.  This
fails on lines containing script labels, since the label-finding code
checks for a line containing only the ":" marker and the label itself
(without any trailing whitespace).

Fix by allowing a label to be terminated by either a NUL or a
whitespace character.

Reported-by: Bovey Christian <Christian.Bovey@chuv.ch>
Signed-off-by: Michael Brown <mcb30@ipxe.org>