upgrading to --current

[Aton A]

I have followed these instructions topdate my  sources and build  the
relevant files

To update your system, you should check /usr/src/UPDATING for any
pre-buildworld steps necessary for your version of the sources and then use
the following procedure:

# make buildworld
# make buildkernel
# make installkernel
# reboot

Then I tried this

After installkernel finishes successfully, you should boot in single user
mode (i.e. using boot -s from the loader prompt). Then run:

# mergemaster -p


but I get an error:
mkdir /var/tmp/temproot: read only filesystem
**FATAL ERROR can not create /var/tmp/temproot


Any help here on fixing this?