I was confused because the comments say that G_OS_PARTITION refers to the partition that the kernel is on, and I thought the kernel was on my rootfs partition (at any rate, I've got a nitdroid kernel in /boot on mmcblk0p3, and setting G_OS1_PARTITION to 5 gives me the dreaded "Please run a maintanance boot" screen (whereas setting it to 3 merely gives me a black screen)).
No idea; it came with Nemo, I didn't touch it.