busybox mount does not accept "-o strictatime"

The "strictatime" flag is not decoded. This was fixed recently upstream in busybox, see https://bugs.busybox.net/show_bug.cgi?id=5240

Significance: I want to debug a boot-time problem by observing which files have been accessed up to the point of failure during boot. Therefore I edit /etc/default/grub, adding ``rootflags=strictatime'' to the GRUB_CMDLINE_LINUX value; and run `update-grub`; and reboot. Boot then fails because the root filesystem mount fails. (Also tested with a seemingly spurious comma, `rootflags=,strictactime'', which I got from googling -- no difference.)

Found in an Ubuntu 11.10 system, but verified this is still true in current 13.04 developmental source.