streamline_config.pl: add LMC_KEEP to preserve some kconfigs
Sometimes it is useful to preserve batches of configs when making
localmodconfig. For example, I usually don't want any usb and fs
modules to be disabled. Now we can do it by:
$ make LMC_KEEP="drivers/usb:fs" localmodconfig
Signed-off-by: Changbin Du <changbin.du@gmail.com> Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>