kernel: Support kernel options required by systemd

These kernel options are all likely to be widely useful in this modern age, but
are immediately useful for systemd support.

c.f. http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/sys-apps/systemd/systemd-9999.ebuild?&view=markup#l118



Adapted from a patch by Adam Porter.

Signed-off-by: Jeff Waugh <jdub@bethesignal.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44929 3c298f89-4303-0410-b956-a3cf2f4a3e73