1. Android build
  - Download original android source code ( android-10.0.0_r25 ) from http://source.android.com   
  ( $repo init -u https://android.googlesource.com/platform/manifest -b android-10.0.0_r25
    $repo sync -cdq -j12 --no-tags
    $repo start android-10.0.0_r25 --all 
  )
  - And, merge the source into the android source code
  - Run following scripts to build android
    a) source build/envsetup.sh
    b) lunch 1
    c) make -j4
  - When you compile the android source code, you have to add google original prebuilt source(toolchain) into the android directory.
  - After build, you can find output at out/target/product/generic

2. Kernel Build  
  - When you compile the kernel source code, you have to add google original "prebuilt" source(toolchain) into the android directory.
  - Run following scripts to build kernel
    a) cd kernel-4.9
    b) mkdir -p out
    c) make -j4 -C ./ O=./out ARCH=arm CROSS_COMPILE=../../prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/bin/arm-linux-androidkernel- CC=../../prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/bin/arm-linux-androidkernel-gcc MTK_DTBO_FEATURE=yes mdh5lm_defconfig headers_install zImage

* "-j4" : The number, 4, is the number of multiple jobs to be invoked simultaneously. 
- After build, you can find the build image(Image.gz) at kernel-4.9/out/arch/arm/boot

