This commit changes the linux source
for VexRiscv and mor1kx to litex-hub/linux.

Since this is the same repository as used
by linux-on-litex-vexriscv and all the LiteX
drivers are included, there is no more need for
additional BUILD_BUILDROOT option.

This also updates rootfs images for both architectures
and adds buildroot configs used to generate them.

This changes the expected boot messages adapting
to the new linux version.

To be reverted after the support for an external DTB file
is brought back to LiteX.