The uboot mapping is only 128KiB (0x20000) and not 2MiB (0x200000).
Dynamically adjust kernel and rootfs mappings based on the
geom_uncompress(4) magic.
This makes the built images more reliable by accepting changes on kernel
size transparently and matches the images built with zrouter and
freebsd-wifi-build.
Tested by: gjb
Approved by: adrian (mentor)
Obtained from: Zrouter