Add target gki419_arm64

The target is for building boot.img with kernel 4.19 and ramdisk.
The kernel is from the kernel prebuilts folder.

Bug: 218629237
Test: lunch gki419_arm64-userdebug; make -j40 dist
Test: cd $ANDROID_PRODUCT_OUT
Test: unpack_bootimg --boot_img boot-4.19-gz.img
Change-Id: I538ae70e50bc21cf253d802f786642e37bae4b50
3 files changed