envsetup.sh: Export LLVM_AOSP_PREBUILTS_VERSION with the current clang version from soong

Needed for KERNEL_CLANG_VERSION if TARGET_KERNEL_CLANG_VERSION hasn't been set by the device config

It's boring to change it on each Google change ...

Change-Id: I1d682611cbf6c60d7ae6227b0b5afde591adf471
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index 21d5e08..140d4a0 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -57,7 +57,7 @@
     KERNEL_CLANG_VERSION := clang-$(TARGET_KERNEL_CLANG_VERSION)
 else
     # Use the default version of clang if TARGET_KERNEL_CLANG_VERSION hasn't been set by the device config
-    KERNEL_CLANG_VERSION := $(LLVM_PREBUILTS_VERSION)
+    KERNEL_CLANG_VERSION := $(LLVM_AOSP_PREBUILTS_VERSION)
 endif
 TARGET_KERNEL_CLANG_PATH ?= $(BUILD_TOP)/prebuilts/clang/host/$(HOST_PREBUILT_TAG)/$(KERNEL_CLANG_VERSION)