Set LLVM and DTC empty. kernel_build() now sets LLVM=1 by default. This change drops LLVM=1 so we can continue using ld.bfd. kernel_build() now sets DTC by default. This change drops DTC because we are using a different Python without pylibfdt. Bug: 236012223 Bug: 164420327 Change-Id: I923545bdb0a579c657d03d928534afc67719ad9a