Learned a lesson the hard way today and thought I would share to prevent others from the same mistake.
On the wiki pages, the page that explains how to build the tool chain suggests using "make -jx" where x is the number of cores on your build platform+2.
So I decided to speed up the kernel and root file system build process with the same trick, substituting "make -j4" for plain make.
Long story short, this breaks the root file system.
I did a clean kernel build from scratch:
git clean -f -x -d
make menuconfig (select default all selections and lose changes)
make vendor_hwselect SYSPTF=/path/to/ptf
make -j4
The resulting zImage booted just fine but gave a kernel panic about being unable to mount root file system.
Repeating the exact steps as above with plain "make" instead of "make -j4" gave a kernel plus root file system that worked fine.
That will teach me to follow directions... I lost a day trying to figure out why my system would not boot anymore :(
