Add build rules for u-boot

Make works with TOOLCHAIN=clang and TOOLCHAIN=gcc-9.2.

test: make u-boot produces a binary that boots with run_qemu.sh
test: verified that all commands use prebuilts from toolchains/

Change-Id: If3a3c13717dbd0f029b4ac94c0746c26397d227f
1 file changed
tree: 35917c388f1fd8930cddfd584732add68f53c1b5
  1. aarch64/
  2. crosvm/
  3. cuttlefish/
  4. kvm-unit-tests/
  5. common.inc
  6. Makefile