kconfig: support DOS line endings

Kconfig doesn't currently handle config files with DOS line endings.
While these are, of course, an abomination, etc, etc, it can be handy
to not have to convert them first.  It's also a tiny patch and even adds
support for lines ending in just \r or even \n\r.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>