kmerge.sh: Don't set USE=build for emerging kernels.

kmerge.sh: Don't set USE=build for emerging kernels.

USE=build for kernels means that they're not going to be built and that
we just want to install the sources. In catalyst, we do also want to
build the kernels, so settings the (badly named) USE flag prevents
installing sys-devel/bc, ultimately leading to the kernel build failing.

Remove USE=build from kmerge.sh and let it install dependencies
necessary for actually building the kernel. Fixes Live CD builds with
kernels that require bc (>= 3.9, I believe). See bug 478244.

Tested-by: Ben Kohler <bkohler@gmail.com>