README: Add Sony README_Xperia

Configuration files can be found in arch/arm64/configs.

  defconfig using in common:

    msmcortex-perf_defconfig

  diffconfigs for each product:

    Xperia XZ Premium G8141       => maple_diffconfig

    Xperia XZ Premium Dual G8142  => maple_dsds_diffconfig

    Xperia XZ1 G8341/G8343        => poplar_diffconfig

    Xperia XZ1 G8342              => poplar_dsds_diffconfig

    Xperia XZ1 Compact G8441      => lilac_diffconfig

How to build your kernel:

  Prerequisites:

    * ramdisk.img - root fs

    * mkbootimg - boot.img generator

    * The ARM cross-compiler

      You can use prebuild executable binary which is included in

      standard Android repository. Please visit to external site.

      In case of this platform, we recommend to use gcc 4.9 or later

      such as aarch64-linux-android-4.9 to avoid known issues.

  Step 1: Build Your Kernel

    $ cd kernel/msm-4.4

    $ export ARCH=arm64

    $ export PATH=<path-to-cross-compiler-executables>:$PATH

      NOTE: Please set the location of the ARM cross-compiler.

    $ export CROSS_COMPILE=<toolchain-prefix>

      NOTE: Please set the prefix of the ARM cross-compiler.

            ex) aarch64-linux-android-

    $ export KBUILD_DIFFCONFIG=maple_diffconfig

      NOTE: Please set a configuration file you want to build.

    $ make msmcortex-perf_defconfig O=./out

    $ make O=./out

    You can see arch/arm64/boot/Image-dtb if you succeed in building.

  Step 2: Assembling the boot.img

    (In the Linux Kernel directory)

    $ mkbootimg \

        --kernel out/arch/arm64/boot/Image.gz-dtb \

        --ramdisk ramdisk.img \

        --cmdline "androidboot.hardware=qcom user_debug=31 msm_rtb.filter=0x237 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 sched_enable_hmp=1 sched_enable_power_aware=1 service_locator.enable=1 swiotlb=2048 androidboot.configfs=true androidboot.usbcontroller=a800000.dwc3 zram.backend=z3fold buildvariant=userdebug" \

        --os_version 9 \

        --os_patch_level 2019-06-01 \

        --base 0x00000000 \

        --kernel_offset 0x00008000 \

        --ramdisk_offset 0x01000000 \

        --tags_offset 0x00000100 \

        --pagesize 4096 \

        -o boot.img