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
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index ef2e879..76281b9 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -27,7 +27,7 @@
     name: "generated_kernel_includes",
 
     // The headers make command
-    cmd: "$(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(TARGET_KERNEL_SOURCE) O=$(genDir) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) headers_install",
+    cmd: "$(KERNEL_MAKE_CMD) $(KERNEL_MAKE_FLAGS) -C $(TARGET_KERNEL_SOURCE) O=$(KERNEL_BUILD_OUT_PREFIX)$(genDir) ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) headers_install",
 
     // Directories that can be imported by a cc_* module generated_headers property
     export_include_dirs: ["usr/include", "usr/techpack/audio/include"],