buildworld/installworld failure 5.2.1

[Steven McMaster]

I am in a spot of bother. 
I have successfully buildworld, buildkernel and installkernel on 5.2.1-RELEASE... but when i came to installworld i ran into a problem. 
I now know that this was a simple problem with the Makefile, but unfortunately i cannot go back.

i am now trying to boot into my old kernel [5.0-RELEASE-p18] and the kernel is crashing with;

"init: /bin/sh on /etc/rc terminated abnormally, going to single user mode"

cool...well at least it gives me a shell.

going by the boot dump... the last message before crashing is 'mounting root from ufs:/dev/ad0s1a'

i run a fsck on ad0s1a and it looks good. any ideas?