am335x-evm config: decrease bootdelay to 1s and mount rootfs RO

A fast boot is important to the beaglebone, so save 2 seconds here by
decreasing bootdelay. This is still plenty time to break into the prompt,
I do that at least once a day.

Mount the rootfs RO by default, this is needed to make fsck succeed
without resorting to an initramfs.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>