config: Use $(abspath .) instead of $(shell pwd)

Change-Id: I7fec28b7cb507918b1b55dd368a74c927a8fc1d2
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index fc60c1c..8978ac9 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -39,7 +39,7 @@
 #                                          is in PATH
 #   USE_CCACHE                         = Enable ccache (global Android flag)
 
-BUILD_TOP := $(shell pwd)
+BUILD_TOP := $(abspath .)
 
 TARGET_AUTO_KDIR := $(shell echo $(TARGET_DEVICE_DIR) | sed -e 's/^device/kernel/g')
 TARGET_KERNEL_SOURCE ?= $(TARGET_AUTO_KDIR)