beinstall: Pass -B to etcupdate

Since commit 246364454fc1 ("etcupdate: Use new buildetc and installetc
targets when available"), beinstall has been much slower for the
etcupdate step, as it's been doing a kernel-toolchain (admittedly
without LLVM itself being built). Given beinstall requires an object
tree to already have been built and just installs it, we can pass -B to
beinstall to reuse that tree rather than build kernel-toolchain in
another one.

Reported by: olivier
Tested by: olivier
MFC after: 1 week