Read it first : https://github.com/peyo-hd/local_manifests

# Build Kernel
 $ cd kernel/rpi
 $ ARCH=arm scripts/kconfig/merge_config.sh arch/arm/configs/bcm2709_defconfig android/configs/android-base.cfg android/configs/android-recommended.cfg
 $ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make zImage
 $ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make dtbs

# Patch frameworks/base
  Do https://github.com/peyo-hd/device_brcm_rpi2/wiki#avoid-logcat-flood
 
# Build Android source
 Continue build with http://source.android.com/source/building.html
 $ source build/envsetup.sh
 $ lunch rpi2-eng
 $ make

# Prepare sd card
 Partitions of the card should be set-up like followings.
  p1 512MB for BOOT : Do fdisk : W95 FAT32(LBA) & Bootable, mkfs.vfat
  p2 512MB for /system : Do fdisk, new primary partition
  p3 512MB for /cache  : Do fdisk, mkfs.ext4
  p4 remainings for /data : Do fdisk, mkfs.ex4
 Set volume label for each partition - system, cache, userdata
  : use -L option of mkfs.ext4, e2label command, or -n option of mkfs.vfat
 
# Write system partition
  $ cd out/target/product/rpi2
  $ sudo dd if=system.img of=/dev/<p2> bs=1M
  
# Boot partition, kernel & ramdisk
  device/brcm/rpi2/boot/* to p1:/
  kernel/rpi/arch/arm/boot/zImage to p1:/
  kernel/rpi/arch/arm/boot/dts/bcm2709-rpi-2-b.dtb to p1:/
  out/target/product/rpi2/ramdisk.img to p1:/