ANDROID: drop KERNEL_DIR setting in build.config.common

And replace many references of ${ROOT_DIR}/common with
${ROOT_DIR}/${KERNEL_DIR}, which makes it a lot easier
to branch off of ACK and update the KERNEL_DIR.

The variable of KERNEL_DIR will be set in build/_setup_env.sh
to the directory of the build config file by default if it's
not set explicitly in the build config file or in environment.

Test: build test with following configs
        common/build.config.cuttlefish.aarch64

Bug: 162785964
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: I5da16e14cf79275622c03daca5c9e9b3e630e948

Author: liuyq

build.config.common

@@ -1,5 +1,4 @@
 BRANCH=android-4.14
-KERNEL_DIR=common
 
 CC=clang
 LD=ld.lld

build.config.cuttlefish.aarch64

@@ -1,5 +1,5 @@
-. ${ROOT_DIR}/common/build.config.common
-. ${ROOT_DIR}/common/build.config.aarch64
+. ${ROOT_DIR}/${KERNEL_DIR}/build.config.common
+. ${ROOT_DIR}/${KERNEL_DIR}/build.config.aarch64
 
 LZ4_RAMDISK=1
 DEFCONFIG=cuttlefish_defconfig

build.config.cuttlefish.x86_64

@@ -1,5 +1,5 @@
-. ${ROOT_DIR}/common/build.config.common
-. ${ROOT_DIR}/common/build.config.x86_64
+. ${ROOT_DIR}/${KERNEL_DIR}/build.config.common
+. ${ROOT_DIR}/${KERNEL_DIR}/build.config.x86_64
 
 LZ4_RAMDISK=1
 DEFCONFIG=x86_64_cuttlefish_defconfig