The kernel that comes with slackware 14.1 (3.10.17) is too large once USB support is added, so it can't boot Linux on 1MB devices. I used instead 3.4.106. According to https://www.kernel.org/ it is an long-term supported version. Whatever that means.
Steps to install:
Download Slackware 14.1 DVD.
Download latest Knoppix DVD.
Install Slackware 32-bit in VirtualBox, A, AP, D, K, L, N disk sets. sda1 as swap (64MB), sda2 as EXT3.
Check that all boots OK.
Change securetty and inittab to add a serial port login
Configure serial support on pipe in Virtual Box config and reboot
Test the serial login works from another virtual machine running Knoppix, connected to the same serial port pipe (Knoppix comes with minicom).
Replace hwclock line in startup scripts as found in previous slackware instructions
Remove stuff about mime and gtk from startup scripts, just wastes time and resource during boot
Download kernel for 3.4.106 from kernel.org
Change timex.h file to adjust PIT_TICK_RATE for bifferboard
Compile bifferboard kernel using the configuration below.
Install the kernel modules with make modules_install
Flash kernel to bifferboard
Set kernel command line console=uart,io,0x3f8 rootfs=/dev/sda2 rootwait
It is working, however seems to use more RAM than Slackware 13.37. Probably worth finding a lighter shell than bash, as bash seems to use 4MB when running.