This is a problem in the build chain, not the build environment.
In older EL6 kernels, there is a default crypto/signature/key.h shipped in the vanilla tarball. In the EL6.1 kernels, this file has been stripped out.
In kernel.spec, we have this block:
gcc -o scripts/bin2c scripts/bin2c.c
scripts/bin2c ksign_def_public_key __initdata <extract.pub >crypto/signature/key.h
However, in lbuild, we have this:
sed -i -e 's/^\(%define signmodules \).*/\10/' \
So we're explicitly disabling module signing, therefore the header is never generated, and builds fail. I'm not sure what the correct solution is, but this shows that it's not a problem with the build node OS.