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/config/BoardConfigSoong.mk b/config/BoardConfigSoong.mk
index 5d36b2c..24183bf 100644
--- a/config/BoardConfigSoong.mk
+++ b/config/BoardConfigSoong.mk
@@ -1,6 +1,7 @@
 # Add variables that we wish to make available to soong here.
 EXPORT_TO_SOONG := \
     KERNEL_ARCH \
+    KERNEL_BUILD_OUT_PREFIX \
     KERNEL_CROSS_COMPILE \
     KERNEL_MAKE_CMD \
     KERNEL_MAKE_FLAGS \