I had this problem recently and found an old message from the archives. So for all the FreeBSD new
builders -- just wanted to update it with the solution I found:
Make sure your development freebsd system has the include files for rebuilding the boot loader:
Configure Do post-install configuration of FreeBSD
Distributions Install Additional
Src Sources for everything
Then pick your ports media.
Excerpt from old post:
Need help with PXEBoot.
section 3 Bootstrap Setup
# rm -rf /usr/obj/*
# cd /usr/src/sys/boot