Rename vendor-ramdisk to vendor_ramdisk

Rename to get consistent ramdisk directory naming in
out/target/product/<name>:
  debug_ramdisk
  ramdisk
  vendor_debug_ramdisk
  vendor_ramdisk

Test: build and inspect out/target/product/<name>
Change-Id: Ib86e6509bd910e8c95bcb6e3f7bb639ebbfe3758