sys-deve/bc is required to compile the kernel (at least with kernel 4.1), so it might be a good idea to include that in the stage3 tarball too? (my installation was not internet-connected, so i had to manually tediously transfer the required distfile and emerge the package that way, offline :s.)
kernel-2.eclass already has: RDEPEND="!build? ( >=sys-libs/ncurses-5.2 sys-devel/make dev-lang/perl sys-devel/bc <<<<<< )" If you install sources of kernel without using package manager, then you should also manually install dependencies :) .
this has been suggested and rejected numerous times. if you are transfering files offline then get the bc distfile with the kernel.
