kernel: Use absolute path for kernel O= argument

The kernel make command is executed in the build root. Without absolute
out paths, kernel out start with "out/" meaning it would be created in
the kernel source. Add BUILD_ROOT to the argument to ensure it's always
made in the actual out/ dir.

Change-Id: I425134a893d4d5c9f31efccaf75e153b5462b76c
4 files changed