[build] remove build errors and create build.log

How to build kernel
1. cd kernel
2. make distclean
3. make .config by using proper boot mode config
  (either one of below two options)
- make ARCH=arm odroidu2_android_sdmmc_defconfig ====> ODROID-U2 (SD Boot)
- make ARCH=arm odroidu2_android_emmc_defconfig ====> ODROID-U2 (eMMC Boot)
4. make ARCH=arm CROSS_COMPILE=arm-none-eabi-
5. zImage will be generated under android/kernel/arch/arm/boot/

How to build android
1. build_android.sh odroidu2
3. build log will be written on the build.log
2. ramdisk-uboot.img and system.img will be generated
  under out/target/product/odroidu2/