I want to boot from usb mass storage device /dev/sda

Hi. I have an external hard drive. It is usb.

I can plug it in and mount it…

But I want its partion to be / so that running
the browser will not be so painfully slow, I want
the browser to be using the hard drive for its cachel.

Not yet, because the usb command is not yet supported under uboot.

One option until then might be to have /boot on the current boot device and everything else on a USB drive.