Revert "build.sh: Fix boot image download to current branch."
Revert "build.sh: Fix boot image download to current branch."

This reverts commit ec490587e7e4413736e25104cfd9be3da551c191.

I didn't read the code, but the odds of a random branch working with master boot image are pretty low.

tgunr/factor,bpollack/factor,bpollack/factor,tgunr/factor,mrjbq7/factor,factor/factor,bpollack/factor,mrjbq7/factor,tgunr/factor,factor/factor,tgunr/factor,mrjbq7/factor,bpollack/factor,bpollack/factor,factor/factor,mrjbq7/factor,bpollack/factor,factor/factor,mrjbq7/factor,tgunr/factor,factor/factor,mrjbq7/factor,tgunr/factor,factor/factor,bpollack/factor